http://www.naturesperfectpets.com/how-to-stop-a-dog-from-chewing-his-paws/

One prime example is that of dogs chewing paws, which is something most pets will be seen performing from time to time and oftentimes without any clear reason. When a dog chews or nibbles away at any area of his body, it's frequently the canine equivalent of a human being scratching an itch.