Probably allergies
My wife's Cavalier King Charles has suffered from allergies all of his life. His worst is grass... when he goes out, it gets between his toes and drives him into a flurry of licking. He licks so much that his paw fur has darkened from saliva and he occasionally gets raw spots. Benadryl used to give him some relief, but not so much anymore. The most useful treatment we've found is to wash his paws with soap & water after he goes out.
He's no dummy.... he's learned to hate the outside & spends very little time there.