Why does my baby chew on his hands?

Exploration. Babies explore their world with their mouths. Also, their hands are the first things they can get to their mouths. The two fit hand in glove to create an important developmental milestone. It can also be a sign of teething, and something infants do to soothe themselves.
Teething. Babies like to chew to ease the gum soreness. Their hands are easily available to chew on.