Why does your dog bark at some dogs but ignore others?

One dog gets a bark and a lunge. The next one gets a sniff and a wagging tail. To the owner holding the leash, it looks like a mood, or a bad day, or plain stubbornness. It is none of those things. Your dog is reading information you cannot see, and reacting to a specific combination of signals rather than to the idea of another dog in general.

Dogs communicate almost entirely through body posture, movement, and scent. Before any sound is made, your dog has already processed the other animal's gait, ear position, tail height, eye contact, and the chemical signals drifting off its body. A dog approaching with a stiff, upright posture, a high tail, and a direct forward gaze is broadcasting a very different message from one that curves its body, drops its tail, and looks away. Your dog reads the first as a challenge and the second as neutral or friendly. The bark, when it comes, is a response to that read, not to the dog itself.

Dogs that grew up with limited exposure to other dogs during the socialisation window, roughly three to twelve weeks of age, often have a narrower template for what another dog is supposed to look and smell like. A dog that met only similar-sized, familiar-breed animals as a puppy may react to a very large dog, a dog with unusual proportions, or a dog that moves differently, such as a sighthound at full extension, as something that does not quite fit the category. The reaction is not aggression in the clinical sense. It is uncertainty expressed loudly.

A single bad encounter leaves a trace. If your dog was bowled over by a boisterous Labrador at six months, it may now react to any large, bouncy, forward-moving dog before that dog has done anything at all. This is associative learning: the dog generalised from one event to a category of physical features. The same mechanism works in reverse. A dog that had consistently good early meetings with calm, well-socialised dogs tends to give unknown dogs more benefit of the doubt. The bark is often a prediction based on memory, not a judgment of the individual animal in front of it.

Many dogs that bark at other dogs on a walk are perfectly fine off-leash at a park. The leash changes the picture in two ways. First, it removes the option to move away, which is a dog's preferred response to something it finds uncomfortable. When the escape route is gone, the dog escalates to a louder signal. Second, tension travels down the lead. If you tighten your grip the moment you see another dog approaching, your dog feels that physical change and reads it as confirmation that something worth reacting to is coming. The bark becomes a loop: you anticipate the bark, you tighten, the dog feels the tightening, the dog barks.

Reactivity that is getting worse, that involves snapping or redirected biting toward the owner, or that makes daily walks unmanageable is not a temperament quirk to wait out. A qualified trainer or veterinary behaviourist can assess whether the root is under-socialisation, fear, frustration, or a pain-related sensitivity that makes the dog more easily startled. Pain from a joint or a skin condition can lower a dog's threshold for everything, including other dogs, and a vet check before any behaviour programme is always worth doing. The behaviour your dog shows on the street is information. The job is to read it as carefully as your dog reads the animal coming toward it.