There are several reasons behind humping. It may be a case of sexual/mating behavior, especially if the dog is not neutered. Another reason may be that your dog is asserting his dominance over you. Many animals, including dogs, hump as a way to show their place in a pack. It could also be a play/emotional behavior. Sometimes, excitement or stress can cause a dog to hump. If your dog does this frequently as an emotional outlet, it is possible it is a compulsion.