The thing about dogs on leash and showing aggresion is that the dog has no where to go and therefore does the only thing they can do to defend themselves. Dogs tend to go through four stages when faced with things that might upset them, there's the Flee stage when the dog tries to escape what is bothering them, the Freeze where the dog stops and refuses to move hoping that that can get them out of the situation, the Fight stage where the dog feels that they have no other choice but to strike out at the situation and finally the frenzy where the dog totally losses their mind, that of course can be shown with barking, jumping, whinning...really it's when your dog is so high that nothing seems to get through to them.

A lot of the time what you see when a dog is on leash is a totally passing over of the first two stages and they go straight to one of the final stages. That of course happens because they can't flee when they're on leash and freezing won't help when the thing they have issue with continues to advance.

Also a lot of dogs consider the area that they can reach when on leash their "protection zone", think of it like when your dog is in your yard or your house and a stranger approaches of course the dog would send up the alarm and try to protect their turf. It's the same when the dog is on leash they're trying to protect you and their area.

Because of the "4 F's" and the protection zone it can sometimes be a better idea to let two dogs meet for the first time in a controlled environment where they can be off leash and both have an opportunity to get of the situation if need be.

Some things that might help with this is to work on something called an emergency u-turn. So when you and your dog are coming up to a situation that you think the dog might not be able to handle (that can be anything from a small animal, another person, or another dog) make a sound or something to get the dogs attention even a turn command or something and turn and head in the other direction. In other words if you don't think that your dog can handle it try not to put your dog in to the situation. Or if you see another dog approaching get your dog in to a sit and have them focus on you until the other dog is past. Whatever you do though do not yank back on the leash as aggression breeds aggression, you yanking back on the leash might make the dog think that there is something that they need to be worried and defensive about and in the long run it could make things worse.

Hope that helps some what.