It's probably because he likes you and wants to play with you. Puppies do a lot of "play fighting" (some married couples do the same thing) and they romp around biting each other. Go watch them in any pet shop and you'll see they all do it. The problem is that puppies' teeth are very sharp (more so than when they're older) and they hurt.
When my dog was small and wanted to bite me I just picked her up and fondled her for a bit, keeping my hands away from her teeth. You could try it.
