I'd like to share something that can prevent or change aggression/fear in dogs, especially if you start at a young age.
Young puppies typically act so wonderful around other puppies. We all think that taking them to a class at Petsmart is exactly what needs to be done, and
it is. I always recommend that to my clients. Puppy socialization is critical. We need to get our babies out and about to learn how to greet other dogs and play, right? However, once they are moving into adolescence, sometimes my clients contact me because their wonderful snuggly puppies are now acting like jerks or they are seeing fearful behavior. This is very common with adolescents, and you can affect this positively in a big way.
I want you to watch this very important six minute video and think about what you are doing with your dog now. I have personally dealt with leash aggression, fearful behaviors, and barrier guarding, and if you've been to my house, you may have seen it in action. What this trainer has to say is imperative to how your puppy develops over the next couple of years, and the way you handle him/her around other people and dogs greatly affects how well they behave around others.
Don't think for a moment that this same behavior isn't going to happen to you, because it eventually will if you have dogs in your home. I love how he describes our training response as a "reprimand sandwich with reward bread on each side". My husband still sometimes doesn't understand why I package my response with a reward when we deal with leash and barrier issues. He sees it as "rewarding bad behavior"; however, properly timed and executed, this handling as a trainer can save you a lot of problems down the road.
Dr. Sophia Yin also has some wonderful videos on her site to help you avoid and deal with aggression and fear. Please go there when you have some time and look over her free, short videos as appropriate. You will have a better future with
your puppy if you invest the time now. http://drsophiayin.com/about