From what I have read there are two things that will generally cause a dog tol ick it's paws.

One is an obseeive behavior. It there was a paw injury the dog may have licked it then to help it heal. That feeling provided comfort, so the dog will continue to lick when stressed to try and find that same comfort, and it could turn into an obsessive behavior. It may also be the dogs way of dealing with stress...seperation anxiety, etc.

The second reson, is like Kay said, allergies. Sometimes, licking the paws is an indication of a food allergy. Watch for other signs of allergies such as congestion, itchiness, ear infections, etc.

That doesn't neccesarily mean there is something rong with your dog it he licks his paws...it may just be habit, like peple chewing their fingernails.