Your Puppy is Nipping/Biting too hard
Why it is a Problem?
He could hurt children or even you. Sure, as a puppy, his bite is fairly easy, but as an adult, the same pressure applied can break skin and bones. If he does this to a neighbor child, the authorities in many cities will even put him down, no questions asked.
Why Puppy is Doing this?
Puppies like to play. This is natural. And puppies do not have hands with opposable thumbs. We often forget that much of the grabbing done by dogs is done with their mouths. In fact, many breeds have been bred to use their biting abilities in various ways that may cause them to bite too hard. For instance, many terriers were bred to catch and kill rats. Some dogs were bred to herd cattle and sheep, using their teeth to nip at the heels of these creatures in order to force them into compliance. Others have been bred to carry objects in their teeth or lead people in this way. Thus, nipping/biting is very natural to puppies.
What You Can Do?
We can largely ignore the habits which have been developed in dogs through centuries of breeding and focus on the playing aspects of this problem. Why?
Because regardless of the reason your puppy is nipping/biting in a way that displeases you, the training is the same. Most undesired nipping takes place during play (at least for puppies), so this is the best time to train him.
Keep in mind that the best way to change behavior is to reward that which is wanted and ignore that which is not. This is especially so for biting/nipping.
Puppies play by biting each other. They tend to get very rough. In fact, two puppies playing will escalate the play until one gets hurt and stops the play. Often, a yelp is followed by withdrawal from the activity. This signals to the other that he has gone too far and he will generally learn how rough he can be with his playmate.
So if he nips at you and you strike or grab him roughly, you are signaling that you want to play rough. He will respond in kind, escalating what in his mind is play. So do the opposite.
If your puppy nips at you, withdraw for a brief time. Twenty to thirty seconds is plenty. You could even yelp as if he hurt you. After the brief time away, if your puppy has stopped nipping, reward the stopped behavior with a treat. This will reinforce that he is not to bite.
Finally, be sure to let your entire family and visitors know what to avoid when playing with puppy. You can do everything right to stop the behavior and have another come behind you reinforcing the biting without knowing it.
Remember, puppy wants to play and if the form of play he chooses is not to your liking, just stop the play. He will get the idea.