Many dogs who haven’t had ample socialization as puppies become stressed when they see other dogs. This may create tension for the owner as well, who may fear a dog fight. Unfortunately, this can become a self-fulfilling prophesy because when the owner becomes tense and pulls back on the leash, the dog responds with even more aggression.
It is possible to change this behavior, but it takes some time and training. While pulling back on your dog and telling him to stop may seem like the right thing to do, it will only increase the dog’s stressful reaction to other dogs. A better option is to give a verbal cue such as "leave it," and then distract the dog and get him to turn to you. Reward your dog with a treat or praise when he does not act aggressively. You will need to do this when the other dog is still in the distance so your dog has just begun to respond. In the beginning, you will need to say “leave it” and go the other way to avoid close contact. But with practice, you should be able to get closer to the other dogs. Keep your voice confident and soft while distracting your dog so that he listens to you and feels your confidence. Arrange to have a friend surprise your dog on his walk with another dog he will recognize so you can practice your distraction technique. With much repetition you should see an improvement over a couple of weeks.
If you find these techniques difficult to use or if the dog has exhibited serious behavior issues resulting in dog fights, consult a professional animal behaviorist for assistance.