I want to foward this from my trainer Natalie McMahon, CPDT, Owner Urban Dawgy:

Creating Leash Aggression

It's one of the easiest things to teach a puppy. Especially when we usually don't even know its happening.

It is so much fun to take a sweet and adorable puppy for a walk around the block. Everybody wants to say "hi" and puppy is happy to oblige. Puppy works hard to make sure everybody gets the chance to pet him. Puppy learns that pulling takes him where he wants to go and it's always fun when you get there.

Shortly, the pulling combined with the choking pup isn't so cute anymore. Now puppy must learn to "heel". So every time he launches out to go see a friend (which always worked in the past by the way) he is met with a leash correction, verbal correction and in some cases a harsher physical correction. Each time we deliver one or more of these options, puppy learns that it's not so nice to see people or dogs when we are out for a walk. In the mind of a puppy, every time he sees a person or a dog approaching, there is trouble coming. The puppy doesn't understand that it has anything to do with his actions. He is doing what puppies do naturally. As puppy gets older, he learns how to tell people or dogs to go away so he doesn't get in trouble. The leash pulling and collar choking continues but now we have added in lunging, growling, and barking. Leash aggression is born.

The corrections continue because now his behavior is getting worse. We don't like the leash pulling but we really don't like it when puppy looks like he's going to attack. It's embarrassing and can be dangerous. Because the corrections are continuing and even increasing in severity, the aggression often gets worse.

So what can be done? Well, the first thing is to understand that puppy doesn't see things quite the way you do and leash walking is not a natural behavior for dogs. Second, go find a trainer that can help you teach puppy that walking WITH you is the only way to go. The earlier training begins the more ingrained the behaviors will be. Teaching the desired behaviors rather than punishing the undesired ones will build a trusting relationship.