Reactive behaviors is a common issue that I am called to help with for clients. The term reactivity is not a well-defined term but, in general, it means that we are dealing with an animal that becomes difficult to control when presented with the stimulus of people or other dogs. The source of this behavior can be from a place of aggression or simply a place of excitement from the animal. In either situation, the basics of the treatment are the same: desensitize and redirect.
First, we work with the pup in a controlled environment to get them used to seeing other dogs or people. It is easy to accidentally get the animal past threshold when trying to accomplish this. The term past threshold simply means that the animal is unable to reason at this point. They are ignoring everything except the thing that sent them to this state. This is not a place where the mind can learn. So we work with the stimulus at a level that the animal can register other things in their environment and wish to eat. If the dog his threshold when another dog is 100 feet away, then we work at 120 feet. As their reaction improves at 120 feet away, we slowly decrease the distance and continue the exercises closer and closer.
While we work on desensitization we also work on giving the animal something to do during these situations. Generally, this is a form of redirection such as targeting a hand or looking at the handler. This causes them to make a choice: pay attention to the stimulus or pay attention to something that will bring me a reinforcer. With enough consistency, the redirection behavior will become almost reflexive when presented with another dog or person.