For puppies, this behavior usually takes on one of two forms. Excitement urination is the result of infant muscles that simply cannot hold their urine when the pup gets excited. Rocky's behavior is partially excitement urination. He can get so excited when he sees his owner that he temporarily loses control of his bladder. The vast majority of dogs simply outgrow this problem as they become stronger and gain control of their muscles.
Submissive urination falls into a completely different category.
Dogs have several behaviors designed to reduce violence between them. When challenged, a submissive dog must display some or all of these behaviors to display its lower status and to prevent an attack. Submissive urination is most commonly offered in this type of greeting. By wetting, the dog is merely acknowledging the other dog's superiority. While all dogs are capable of offering this behavior, few are as bad as Rocky.