Why do dogs hump?

I volunteer with Louisville Metro Animal Services, the city shelter, walking dogs and taking them on field trips (outings outside of the shelter). A couple of weekends ago, we attended Hammer Lou, the city’s largest adoption event. I was paired with Cannonball, a 20ish pound, 6ish month old, black and white recently neutered pup. This cutie did not stop humping me for an hour an a half!    Humping is when a dog gets on another dog, person, or item with front legs around another dog,