My dog, who is normally really good with other dogs, recently started being really aggressive when we would take him out for walks. At the dog park, she barely cares about the other dogs, but once we put him on a leash, he would bark, growl, and jump at any dogs we would walk by. After some research, we found these great tips to help take care of this problem:
1) Dogs get aggressive on leash because they are forced to approach other dogs head on rather than from the side or by smelling their bums first. This automatically makes them feel confrontational. Try then not to have your dog quickly walk up to another dog head on.
2) Dogs equate this situation as bad because you immediately tug them away from the other dog and punish them for their bad behavior. To fix this, make each situation where they see another dog a good thing. We've been doing this by carrying around treats and each time my dog spots another dog and starts to get excited or agitated, we distract him with treats and reward him for seeing that other dog. Eventually, this should make your dog equate seeing other dogs with reward.