Humping is never a dominance thing because dogs do not try to dominate humans or other dogs. Not even wild wolves function in this fashion as previously thought.

Humping can be caused by many things. I've known long since neutered males who hump for England....I live in England, lol...then I know unneutered ones who never humped....so it isn't always hormonal either.

Saying that, one of my bitches started trying it with me when we were fussing each other on the floor....it was almost like an affection thing but she hasn't done it since being spayed so it was probably also hormonal. She's a very intense bitch when it comes to emotions.

I'm sure some other dogs hump purely out of enjoyment or as a gesture of affection even....even stress can cause humping because it is a self rewarding behaviour that might help to relieve the stress.