Here’s an example of two dogs going over threshold because they got too close to each other.

A reactive dog is one whose responses to certain stimuli are much greater than we think necessary. A stereotypical reactive dog is an adolescent, 18-30 months of age, but reactivity can start as early as seven months. Often they are herding dogs, mixes and rescues, but they come in all breeds and mixes, all sizes and shapes, all backgrounds, and all ages. The most important factors are socialization, genetics, and stress.

Here is an excerpt from Ali Browns’ book, Scaredy Dog!, Understanding and Rehabilitating Your Reactive Dog:

What happens when you realize there’s another dog in the vicinity? Do you cringe? Do you get that sinking feeling because you know what’s about to happen? Do you tighten up on the leash in anticipation of the lunging? Do you hold your breath? What happens when your dog starts his “aggressive” routine? Do you pull back on the leash? Do you yell at your dog? Or do you tell him “it’s OK?” Do you try to hold him back physically? Believe it or not, some of these things are reinforcing to your dog. But ALL of them affect change in your dog’s behavior. Think about it: every time he sees another dog and he’s on his leash, he gets a leash correction. This teaches your dog that there’s an association between dogs and pain/discomfort in his neck, or at the very least, that his person becomes very upset when there’s another dog nearby. There must be a good reason to be leery of those other dogs, then. No wonder he reacts to other dogs!

Well, what about the yelling? Or telling him “it’s OK”? Why don’t they work? In order to understand what’s going on, we need to learn a few more concepts.

We can make our dog’s behavior change in three different directions. The first is the one we learned earlier in the book: to enhance the behavior through positive reinforcement. That would be something like “it’s OK Rover; that doggy’s not going to hurt you.” While it’s clear to us what we mean to say, our dogs don’t understand English. But although he doesn’t understand the words, the tone is one he’s heard before: soothing, encouraging. “Oh,” he thinks. “She wants me to keep doing this! Got it!” And so your dog steps up his efforts to drive the other dog away.

The second way we could change the behavior is to diminish the behavior through the use of positive punishment. Positive punishment is anything which follows a behavior which is aversive to the dog and which serves to decrease the chances that that behavior will happen again. So, you think, I’ll just yell at him. That’ll stop him. Think again. While your dog is barking, he’s barely hearing your voice behind him. And he doesn’t know what you’re saying, but you must be encouraging him. After all, you’re the “big guns” behind him. While you think you’re punishing him, he thinks your salutations are supporting him. This is, then, positive reinforcement, which enhances the behavior, rather than positive punishment, which diminishes it.

Positive punishment can be effective if it’s severe enough. You should only need to use positive punishment one or two times in order to make that behavior go away. There are several problems with this. First, are you willing to deal a severe enough punishment that the dog is unwilling to repeat the crime? Most of us are not; it usually involves fear or pain, and we didn’t get a dog so that we can brutalize him. Second, punishing one behavior “successfully” almost always results in the development of other, even less desirable, behaviors.

But the side effects of positive punishment can be even worse. It is important to understand that it is the perception of the dog, not the perception of the human, which dictates whether something is punishing or not. So you may never actually USE positive punishment, but the dog may perceive something in his environment as being punishing. A dog who is the subject of repeated positive punishment will react in a predictable manner. At first, the dog becomes anxious. You may see signs of this anxiety by looking more closely at him. Are his ears moving around a lot, as if to constantly check the direction from which sounds are coming? Is he doing a lot of nose licking and yawning?

If punishment continues, he might start to become fearful. Fear leads to attempts to escape the source of the punishment (you or that particular environment). The dog may go from being only a danger to himself, to being a danger to others because he may try to run away, possibly getting hit by a car. If the dog is inside, he may try to run and hide under a table or a bed, or run to his crate or to the basement. The next level of reactivity to punishment is avoidance of the source of punishment altogether. This is where your dog really starts to “blow you off” or runs away from the situation…