What are all the reasons dogs mound each other?

In younger dogs, they aren't actually SHOWING dominance but TESTING their dominance. The dog doing the is seeing just how far they can push the dog being humped. This is why it often occurs during excitatory play. Should the dog being humped issue a correction strong enough to get the message across, the "humper" now knows his boundaries. If the "humpee" does not correct, then the "humper" knows he's the boss.