haha yes, such a nice topic! actually humping has nothing to do with dominance. or sex actually (unless the dog is an intact male). its actually a part of normal play. (my mentor likes to equate humping to a big hug ) almost all dogs do it, of all ages and genders.

there are 4 aspects to dog play- food, fight, flight, and sex. basically play is "practice" for these 4 necessities of survival in dogs. so, as embarrassing as it is to us humans, its totally normal and fine between dogs. often it happens when the dog is excited, like if theres a lot of running around or a lot of action. some dogs dont like it and will let the humper know, but many other dogs could care less.

basically my suggestion to people is to either let them do it, or just interrupt them casually. but never scold or punish them for doing it (even though its totally embarrassing, especially at the dog park! haha). crazy dogs!