When walking a dog who reacts, such as barking aggressively and/or lunging, when he/she sees, passes or hears another dog or dogs, positive associations can affect the dog's behavior. So when the dog begins to lunge or growl, pull out some good food such as pieces of cheese or treats he can't resist, tell him to sit, and give him some, each time this happens (you can be generous with the treats so the dog will remember). Soon the dog should begin to associate other dogs with pleasant things, the food or treats; the unwanted behavior will diminish, or at least become manageable. A dog's emotional associations can be manipulated; we can take good advantage of this. This is known as classical conditioning. Dogs are different than people in that dogs can't think about how they feel, they just feel it; humans can think about their feelings intellectually and control them. Dogs can't control their feelings; they just have a reaction. They may have had a bad experience with a dog, or have been undersocialized.