One of the most important aspects of puppy training is teaching your dog not to bite.  This isn’t easy but you have to be firm and stick to your guns. When playing young dogs will automatically nip and gnaw at your hand.  This is natural behaviour from the dog as when dogs play together they play with their mouths. Biting is a natural thing for a dog.  When teaching your labrador retriever not to bite stroke him with one hand while offering something which is acceptable to be bitten with the other hand, something like a chew toy or a raw hide treat. Your dog will soon realise that hands aren’t there for biting.