Why does my dog play bite me?

If this is a puppy it's natural for him to want to bite because he's teething. It's important to break him of this habit. When he bite make a loud OW! Noise. Ingore her for ten to twenty seconds, then resume play.

Dogs bite things with their mouth because that is how they interact with the world.

When my dog was a puppy, I wanted him to learn to not bite hard, but that mouthing was OK (some people want to teach no mouthing at all, that's ok too). I yelled OW! when he bit too hard and put him down for 30 or so seconds. I only needed to do this a few times as a puppy (maybe 10 total) and now he is 3.5 years old and if I'm playing rough he will mouth/grab my hands, but has never bit hard.