There are a variety of types and styles of pet training in existence that you can use on your dog. Different styles are appropriate for different dogs and circumstances. One such method is that of classical conditioning.
Everyone has surely heard of Pavlov and his dogs? Passive conditioning was discovered by Pavlov when he noticed that if he rang a bell every time he fed his dogs, after a time they would salivate when they heard the bell even if there was no food. The continued pairing of a stimulus (the bell) with a response (salivating because of food) led to the response happening even without the original trigger.
Classical conditioning can be used to help a dog get over a fear of/aggression towards other dogs. A dog that behaves in a negative way around other dogs does so because he has been classically conditioned by one means or another to feel afraid or aggressive when he sees the trigger of another dog.
Classical conditioning can make this trigger elicit another response by means of counter-conditioning.
First, it must be determined how close the other dog can be before the dog being conditioned will respond. This distance is known as the reaction threshold. It is important, so far as possible, to keep the subject dog below his reaction threshold, while ensuring that he does notice the other dog.
As soon as the subject dog has noticed the other dog, but before he has reacted, begin praising the dog and giving it treats. If there is a concern over the dogs weight, simply take some of its daily food to be used as treats.
Over time, the subject dog will begin to associate the arrival of another dog with being given praise and treats as long as the subject dog is calm.
This is not an easy and quick solution to a fearful or aggressive dog, but it does work if the time is put into it.
Conditioned responses may fade over time if they are not reinforced. This does not mean that treats must be given every time the subject dog sees another dog for the rest of its life, but instead that once the dog is at an appropriate level of conditioning and firmly associates other dogs with treats and praise, the owner can begin to give the treats intermittently instead of every time.