Why does my friend do that?

he is 15 (me too) and it's weird but he bites me. He bites my hand, usually for no reason. we have a pretty good relationship so I don't disgust about it but I wonder why he does it. he has a lot of friends (girls) but he doesn't bite them. sometimes he puts his head on my lap or rests on my shoulder and that's probably normal for friendship but I'm still worried what that biting means.
Why does my friend do that?
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