Why My Dog Is Reactive to Certain Dogs, and What Most Owners Miss

Quick answer: Your dog reacts to certain dogs because it is reading something specific in that encounter: how the other dog approaches and looks at it, whether a leash stops it from moving away, what that dog resembles from its past, and how much stress it is already carrying. Fear is the usual engine, and frustration at being held back is the other common one. Before you blame a breed or a color, check the approach, the leash and your dog's day.

Key takeaways:

• In a peer-reviewed survey of 13,715 dogs, fear of other dogs was the most common of the social fears measured, reported in 17% of them.
• Threat displays between dogs are about twice as common on leash as off leash, according to research from Mendel University in Brno.
• The other dog's behavior matters: in dog language, a direct, wide-eyed stare communicates a threat.
• In a 2026 University of Lincoln study of 1,360 dog-reactive dogs, those that also reacted to people, sounds or vehicles were more likely to fall into the higher-risk groups, and reacting to children nearly doubled the odds of landing in the highest-risk group rather than the lowest.
• Pain, aging and stacked-up stress lower tolerance, so a sudden change in which dogs your dog accepts is a reason to see your vet first.

Your dog walks past a dozen dogs without a second look, then loses it at the thirteenth. If you have been asking why your dog is reactive to certain dogs and not others, the honest answer is almost never "no reason." Dogs read each other constantly, and the lunge you see on the sidewalk is the end of a conversation that started well before it.

At Canine Perspective in Chicago, aggression and reactivity are the core of our training work, and finding the triggers is where that work starts. This guide covers the four things that decide which dogs set your dog off, what veterinary and behavioral research says about each, and the parts that are easiest to miss.

In this guide:

• Why is my dog reactive to certain dogs but fine with others?
• What is the other dog doing that sets your dog off?
• Why is my dog only reactive on the leash?
• Does my dog's past explain which dogs it reacts to?
• Could the problem be your dog's day, or its body?
• Does reacting to some dogs mean my dog is dog-aggressive?
• What do most owners miss?
• How do you work out your dog's real pattern?
• What actually changes selective reactivity?

Why is my dog reactive to certain dogs but fine with others?

Because your dog is not reacting to "dogs." It is reacting to a particular mix of signals, setting and its own state, and only some encounters add up to "that dog is a threat" or "can't get to that dog."

Not every dog is meant to like every other dog, and being choosy is normal. Reactivity is the over-the-top part: the American Kennel Club describes it as an excessive level of arousal in response to ordinary, everyday things, like another dog walking past. The word describes intensity, and the AKC notes that fear is a common motivator while frustration or excitement can be behind it too.

Fear is the most common one. In a 2020 study of 13,715 Finnish pet dogs published in Scientific Reports, 17% of dogs showed fear of other dogs, the most common of the fears the study grouped under "fear" (other dogs, strangers and new situations; noise fears were counted separately). Fearful dogs in that study were 3.2 times more often aggressive (the study measured aggression toward people), which the authors read as a sign that aggression is commonly motivated by fear. Cornell University's College of Veterinary Medicine puts it plainly: a reactive dog is usually a fearful dog.

Frustration is the other common engine: a dog that wants to reach another dog and cannot. Either way, which dogs trigger a reaction comes down to four layers:

• The other dog: what it does, how it moves, how it looks and smells.
• The setting: the leash, the distance, and how much room either dog has.
• Your dog's history: early social experience, bad encounters, and what reacting has "won" before.
• Your dog's state: stress already stacked up that day, pain, age, and general arousal.

A reaction usually needs more than one layer at once. That is why the same neighbor's dog can pass quietly on Tuesday and set your dog off on Saturday.

What is the other dog doing that sets your dog off?

Often it is the other dog's behavior, not its breed. Dogs communicate through posture, facial expression, tail and ear position, raised hackles, vocal sounds and scent, as the Merck Veterinary Manual's guide to normal social behavior lays out, and much of that is easy for people to miss.

Behaviors that commonly push a reactive dog over the edge:

• A direct approach with a hard stare. Board-certified veterinary behaviorists writing for VCA Animal Hospitals note that a direct stare communicates a threat, and describe it as the most subtle of the distance-increasing signals. A stiff, forward-leaning dog locked onto yours is saying something your dog understands.
• Pushy, bouncy greeters and puppies. Research on dogs meeting during walks found that adult dogs threatened puppies more often than they threatened other adults, possibly a reaction to the puppies' strong urge to play (Mendel University in Brno).
• Off-leash dogs running straight at you. A charging dog gives your dog no time and no exit; our guide on what to do if an off-leash dog approaches you covers that situation.
• Intact males, for some males. The ASPCA notes that intact male dogs sometimes challenge and fight other males even when no females are around.

Raised hackles are worth reading correctly too. VCA's behaviorists note that hackles signal high emotional arousal, not necessarily dominance or aggression, so they tell you the temperature, not the intent.

The useful distinction here is between a label and a thread. "He hates huskies" is a label. "He reacts to dogs that come straight at him" is a thread you can work with.

Some breeds show fewer signals

A 1997 study in Animal Behaviour counted how many ancestral wolf signaling patterns ten breeds used with each other. The number ranged from two in Cavalier King Charles spaniels to fifteen in Siberian huskies, and tracked how wolf-like each breed looks (Goodwin, Bradshaw and Wickens). The study did not measure how other dogs react, so treat this as an inference, not something the study found: a dog that shows fewer readable signals may be harder for an anxious dog to predict.

Why is my dog only reactive on the leash?

Because a leash takes away your dog's two best options: moving away, and greeting on its own terms. A 2017 paper from Mendel University in Brno, citing the same group's earlier observations of dogs on walks, reports that threat displays were about two times more common between dogs on a leash than off one, and suggests leashed dogs may feel more vulnerable because they cannot run away (Sediva and colleagues).

Owners of reactive dogs see the same thing. In the 2026 University of Lincoln study of dog-reactive dogs, 42% of owners said their dog was less reactive off leash, 3% said more reactive, 19% saw no change, and 35% never let their dog off leash (Van Haevermaet, Soulsbury and Mills). Cornell's vets describe the pattern too: some dogs are fine with other dogs off leash but become reactive on one.

On a Chicago block, the space can make it worse: narrow sidewalks, doorways, parked cars and building hallways where neither dog can put distance between them. VCA's veterinary behaviorists note that dogs that cannot retreat because they are cornered or restrained are more likely to fight.

Two different dogs can look identical at the end of a leash:

• The fearful dog wants the other dog gone. The bark and lunge are an attempt to create distance it cannot get by walking away.
• The frustrated dog wants to get to the other dog and cannot. The AKC notes that dogs who get frustrated when a leash, fence or other barrier blocks their excitement to greet often lack emotional self-control, and the ASPCA warns that an aroused dog held back from something can become aggressive, particularly toward whoever or whatever is holding it back.

That is how a dog ends up growling at a dog it actually wants to meet. The Lincoln researchers found that barking, lunging and whining cluster together as a separate "frustration" component, which the lead researcher, speaking to AAHA's Trends magazine, links to thwarted social contact, leash restraint and blocked access or escape. Frustrated dogs did not necessarily escalate to contact, but that is not something to test on the street. Many dogs are a mix of both, which is why the pattern matters more than the label.

Watch your own end of the leash as well. If the leash snaps tight every time a dog appears, your dog gets an early warning that something is coming and loses the slack it needs to turn away. Where it is safe, keep the leash loose and make distance by moving your feet early, rather than reeling your dog in at the last second.

Does my dog's past explain which dogs it reacts to?

Often, yes. What your dog learned about other dogs, early in life, in one bad moment, or through repetition, shapes which dogs it now treats as a problem.

A bad experience with a look-alike

Fear-related aggression can be aimed at animals that have been linked to an unpleasant or frightening experience, according to VCA's veterinary behaviorists, who add that some dogs learn and generalize from a single experience. A dog that was rushed by a large, dark, heavy-set dog may start reacting to dogs of a similar size, color or movement. It looks like breed prejudice; it is really a memory.

Gaps in early social experience

Puppies focus on other dogs for social interaction between about 3 and 8 weeks of age, and stay most open to new situations until roughly 16 to 20 weeks, according to the Merck Veterinary Manual. The Mendel University paper cites earlier research linking gaps in early social contact with a higher risk of aggression toward other dogs later. In the University of Lincoln study, dogs that came from breeders and were raised in a home from puppyhood were significantly less likely to fall into the higher-risk groups.

None of that is about blame. If your dog came home after that window had closed, the gap was never yours to fill.

Social maturity: "he used to love every dog"

Merck places social maturity at 12 to 36 months of age and notes that this is also when problems with aggression and anxiety develop. A puppy that played with everyone at six months can become choosy at two years. That is normal development, but it is also when patterns set, so act on it early.

The reaction keeps working

This is the easiest one to overlook. VCA's behaviorists explain that when a dog learns aggression succeeds in removing the thing that worried it, the behavior is reinforced. On a walk, the other dog almost always keeps moving away. From your dog's point of view, the lunge worked almost every time.

Could the problem be your dog's day, or its body?

Yes, and it is easy to check last. Sometimes the dog your dog reacted to had very little to do with it.

Stacked stress. The AKC describes trigger stacking as several stressful or scary events landing within a short span, and notes that cortisol takes time to come back down; some dogs carry the effects for days. A dog that already dealt with a delivery truck, a vet visit and a construction crew may have nothing left for the next dog on the block. Our post on the hidden triggers behind aggression goes deeper on how this and pain build.

Pain. The ASPCA puts it simply: an otherwise gentle, friendly dog can behave aggressively when in pain. If the reactivity started suddenly, got worse quickly, or came with other changes in how your dog moves, eats or sleeps, see your vet before you start any training.

Age. Older dogs can become less tolerant for physical reasons. VCA's veterinary behaviorists note that older pets may lose some vision or hearing, fail to notice another animal approaching and startle more easily, and that arthritis can make a mild, existing behavior problem worse. If a dog that was always a little choosy has become reactive in its senior years, start with your vet.

Arousal and structure. Our view, stated on our aggression training page, is that most aggression is rooted in fear, over-arousal, or a total absence of structure. A dog that leaves the house already wound up has less room before it tips over. A calm dog can learn; a frantic one can't.

Does reacting to some dogs mean my dog is dog-aggressive?

Not necessarily, and the loudness of the reaction won't tell you. What matters is what your dog actually does when it reacts.

The AKC separates the two ideas: reactivity is over-the-top arousal, while aggression is threatening or harmful behavior meant to create distance or cause damage. In practice they overlap, and the useful question is how far your dog goes.

The 2026 University of Lincoln study (the paper, also summarized in AAHA's Trends magazine) drew on 1,360 owner responses about dogs that react to other dogs. Biting occurred in 24% of them, and the dogs fell into four groups:

• Low-risk signalers (40%): bark and posture, with very little attempt to make contact.
• Frustrated escalators (18%): very high frustration and heavy posturing, but low to moderate contact.
• Impulsive escalators (32%): willing to make contact without being especially frustrated.
• Risky rapid escalators (10%): high on everything; they escalate fast and follow through.

Three findings matter for you. The loud, lunging dog is not automatically the highest risk. A dog's sex, neuter status, age and body size did not predict which group it fell into. And dogs that also reacted to people, sounds or vehicles were more likely to be in the higher-risk groups; in the paper's own analysis, dogs that reacted to children or to men had nearly twice the odds of being in the highest-risk group rather than the lowest.

Those links are modest, though. The paper's model explained only about 5 to 6% of the variation in which group a dog fell into, so no single fact about your dog settles its risk. If your dog has snapped, nipped, bitten or fought, or reacts to people and children as well as dogs, treat it as a safety issue now, not a quirk. That is the kind of case our dog aggression training is built for.

What do most owners miss?

Put the research together and six blind spots stand out:

1. The lead-up. Subtle signals usually come before bold ones, VCA's behaviorists note, though some dogs skip steps or move through them very fast. The stare, the stiffening and the weight shifting forward are the window where you can still change the outcome.
2. The approach, not the breed. Don't just log what the other dog was. Log what it was doing.
3. The leash and the space. A narrow sidewalk and a tight leash change the whole encounter, for both dogs.
4. The payoff. Nearly every walk-by ends with the other dog leaving, which teaches your dog that reacting works.
5. The day and the body. Stacked stress, pain and age lower your dog's tolerance before it ever sees the trigger.
6. "Fine with dogs he knows" proves less than you think. The ASPCA notes that aggression toward unfamiliar dogs is widespread, while aggression toward household dogs and pets is less common. Getting along with a housemate does not rule out a real problem with strangers.

How do you work out your dog's real pattern?

Keep a simple log for two to three weeks and let the pattern show itself, instead of reconstructing it from memory. Record every reaction, and every near-miss where your dog stiffened but held it together.

For each one, note:

• The other dog: size, build, coat, color, and sex or neuter status if you know it.
• What it was doing: coming straight at you, staring, pulling, walking calmly parallel, playing.
• Distance: where your dog first noticed it, and where it reacted.
• Leashes: was your dog leashed, and was the other dog? How tight was yours?
• The setting: open park, narrow sidewalk, corner, building entrance, hallway, elevator.
• Earlier that day: walks, visitors, vet trips, noise, time alone.
• Your dog's response: stare, stiffen, bark, lunge, snap, and how long it took to settle.

Then read it back looking for the thread. If it is a behavior, like head-on approaches, or a setting, like tight spaces, you have found something specific to work on. If it is everything, everywhere, including people and noises, that points to more generalized reactivity, the pattern linked with higher risk in the Lincoln study, and a reason to get professional help sooner.

What actually changes selective reactivity?

Knowing the pattern, then changing how your dog feels and behaves in that exact situation, starting at a distance where it can still think. Exposure on its own can backfire: VCA's behaviorists note that when an encounter frightens or harms a dog, its fear is likely to get worse.

While you work on it, manage the triggers so your dog stops rehearsing the reaction:

• Give more distance than you think you need, and turn or cross early rather than late.
• Skip on-leash greetings with dogs you don't know.
• Avoid the pinch points your log turned up, such as blind corners and narrow building entrances.
• Don't "test" your dog by walking it up to the kind of dog it reacts to.

Management is not the fix. It stops the problem growing while you do the real work; avoiding every dog forever is not a life for you or your dog.

The real work in our program follows the same order: find the triggers and thresholds, build calm and impulse control before anything else, then carefully re-expose the dog under control so new responses replace the old ones. When we use the e-collar, it's introduced at the lowest level the dog can perceive and paired with commands it already knows, never as punishment. E-collars are debated among trainers and vets, so our methods page lays out exactly how and why. For the step-by-step on safely rebuilding your dog's comfort around other dogs, see our guide to controlled socialization for reactive dogs.

Get professional help now, rather than later, if:

• Your dog has snapped, nipped, bitten or been in a fight.
• Reactions are getting more intense or starting from farther away.
• Your dog also reacts to people, children, sounds or vehicles.
• It takes your dog a long time to settle after an episode.
• The change was sudden. Start with your vet, as the ASPCA advises, to rule out a medical cause.

Frequently asked questions

Why does my dog react to black dogs, or to one breed?

Usually because of a pattern rather than the color or breed itself. Some dogs generalize from a single bad experience to dogs that look similar, VCA's behaviorists note, and as general reasoning, a dark face or a very un-wolf-like body can make signals such as a hard stare harder to read. Your log will tell you whether the real thread is the look, or the way those particular dogs approached.

Can a dog be reactive to just one specific dog?

Yes. It can happen with a dog your dog sees through a fence or window every day, where barrier frustration gets rehearsed over and over, or with a dog linked to a bad experience. Treat that dog like any other trigger: more distance, no forced meetings.

Does neutering stop a dog from reacting to other dogs?

Don't count on it. The ASPCA notes that intact males sometimes challenge other males, but in the University of Lincoln study a dog's sex and neuter status did not predict how risky its reactivity was. Talk to your vet about the medical side, and treat the behavior as behavior.

Will my dog grow out of reacting to certain dogs?

Don't wait for it. Social maturity, at 12 to 36 months, is when aggression problems tend to develop, not fade, and nearly every walk-by that ends with the other dog leaving teaches your dog the reaction worked. The earlier you address the pattern, the less practice it gets.

Is my dog reacting because it's protecting me?

It is possible; the ASPCA lists protective aggression, when a dog believes a family member is in danger. But check the ordinary explanations first: a leash that stops your dog leaving, a dog coming at it head-on, or a dog that is already stressed or sore.

Did I cause my dog's reactivity?

Rarely on your own. Cornell lists genetics, a lack of socialization, prior bad experiences and a lack of training as causes, and much of that can happen before a dog ever comes home. What you control is what happens next.

Get an honest read on your dog's pattern

If your dog reacts to some dogs and you can't find the thread, a consultation gets you an experienced read on what is driving it and what realistic progress looks like. We look at your dog's triggers, thresholds and history, and tell you straight what we would do. We won't promise a cure; we will give you a real plan and show you how to carry it out.

Book a consultation with Canine Perspective, or call our training line at (872) 294-4940.