Puppy aggression is a serious problem that can often be easily prevented. Once your puppy has shown aggression, it can take months or years to cure, but it can take only weeks to prevent.
Socialization Period
The most common forms of puppy aggression are caused by fear and frustration. Fear can often be prevented by exposing young puppies to people and things they will encounter throughout their life.
All of your puppy's neural pathways will be almost completely formed by the time he is 16 weeks old. Anything that he sees after that time may be foreign and scary to him, causing him to react with fear barking, growling and possibly biting. To prevent this, he must spend time meeting other people and dogs during his first 16 weeks.
Socialization Checklist
Every dog's socialization needs are a little different depending on the lifestyle that you lead. For example, a country dog may need exposure to horses and cows while a city dog needs exposure to crowds and heavy traffic. Think of all the things in your lifestyle that you need your puppy to accept such as your children, other dogs, the gardener, the mailman, elderly people etc.
Variety is crucial. Your puppy needs to be exposed to tall men, short men, fat men, men with limps, men with beards, men wearing hats, men jogging, etc. The same is true for women, children and elderly. He also needs to be exposed to bicycles, skateboards, construction, lawnmowers and anything else that you might encounter on your daily walks.
Proper Socialization
However, it's not as simple as just exposing your dog to these new stimuli. The experiences also need to be positive. You should never force your puppy to do something that makes him nervous. That will make him believe that you can't be trusted and that he needs to protect himself, which he will do by barking or biting.
If your puppy seems nervous, don't force him any closer. Just sit at a safe distance and let him observe. Use lots of treats in the interactions so he is excited to see the bearded man, jogging man and man with hat. Offer him treats yourself and ask if new people would like to offer him one as well.
Keep your sessions short, so they always end before they get scary. If your puppy seems tired or stressed, remove him from the situation and try something a little less stressful next time.
Also, only allow your puppy to greet if he is behaving properly. Socialization shouldn't be an opportunity for your puppy to practice jumping and nipping. Don't punish him for these but calmly hold him back on leash until he is calm enough to greet politely. If he starts to jump or nip, pull him back and wait for him to calm down again. This will help him learn to calm himself, which will greatly reduce the chances for aggression from frustration.
Socialization is an important part of your puppy's development. Without it, new experiences can become very stressful for both you and your puppy. With it, your puppy will become a healthy, well-adjusted dog who can adapt to new situations easily.