At 12–16 weeks your dog will begin to test you to see who the packleader is going to be.
He’ll begin to bite you, in play or as a real challenge to your authority. Such behavior is natural in the pack and not necessarily undesirable. It is important, at this stage, that you establish your position as pack leader, and not just another sibling. Other behaviors, such as grabbing at the leash, will be observed, and all are attempts to dominate you.
Biting, in particular though, should always be discouraged. Therefore, you should not wrestle or play tug of war. Such play is aggressive–inducing. What you see as a fun game may be perceived by your dog as a situation in which he has been allowed to dominate. Wrestling, of course, communicates to your puppy that he is allowed to bite you. Tug of war sets you up in a dominance confrontation over an object. He learns that he can keep objects away from you. During tug of war games, puppies will often growl. Growling is a dominance vocalization, designed to warn another pack member that they better not confront the growler or he will bite. Puppies see these games as situations in which they have been allowed to dominate. They do not understand that these are games designed by humans to entertain them.
Continue to play with your dog during this period, but make it clear that no mouthing of your body is allowed and when your dog does mouth, you respond with a quick and sharp “NO!” or “No Bite!” Play that does not get rough is best. If you cannot keep the dog from getting overly excited during a game and he persists in biting at you, don’t play that way.
For these reasons, this is the stage when serious training should begin. Training establishes your pack leadership in a manner that your puppy will understand. By training your puppy, you will learn how to get him to respond to commands designed to show that you are in charge.