Why does my dog chew on me?

I am assuming this is a puppy. Regardless you need to train them that this is not acceptable behavior. Teach your puppy bite inhibination: make a loud noise when the puppy bites you and then give him a treat when he backs off. Always give your puppy an alternative toy to chew. Make sure you make it clear to the puppy that it is not acceptable to chew or bite you. You can also try challenging toys like kongs that you can put peanut butter or treats in.