Dog behavior is determined by two primary factors; early training and socialization as puppies and the temperament of breed and parents.
Genetics is an important aspect of dog character, but with proper obedience training at an early age most puppy aggression can be averted. Young pups are at a stage when they learn how to react to new situations and environments. What they learn at this time will be difficult to 'undo' later on. Young dogs are relatively helpless and aware of their dependence on their owner for basic needs.
Chances are the worst 'aggression' type behavior you will see is mouthing or playful biting. Occasionally a young puppy will bite too hard while playing. This is not cause for alarm, although should be gently corrected. He simply isn't aware of bite aversion yet. You can respond by reacting with a loud shrill scream of pain (genuine or otherwise) to indicate the inappropriate bite behavior. Ignore his desire to continue to play. Puppy's won't play with other puppies who bite too hard which is an strong signal to your pup about his biting.
The most important and fundamental key to successful dog training is to reward your dog with love, praise, attention or food (whatever he responds well to) for appropriate behavior. “Punishing” your dog is never the right answers. Behavior modification requires firm but consistent response from you, but it is never necessary to hit or scream angrily. If you react properly, you will get proper response given time and patience.
For example, if you discover your puppy chewing on a dish towel and approach him, he may playfully run away with the towel. Don't yell or scream, and don't chase him. This will only reinforce the bad behavior because he thinks this is a fun new game that involves destroying your property. Try either stopping and coaxing him to you or slowly run in the opposite direction. He may run toward you. When he reaches you, gently take the object from him while repeating the command “drop it” and replace the towel with one of his toys. Praise him for the exchange.
If you are consistent with this he will soon learn that he is rewarded when he behaves properly.
