
Haha, I know its undesirable behaviour, but it IS funny. :)
Nellie - my 18 month old terrier bitch, who was spayed two months ago - has taken to frantically humping my husbands leg whenever he lays on the floor. We do stop her, but usually my husband is so helpless with laughter at the expression on her little face that I have to join in. We call her "Humphrey".
I have no idea why she does it, but I doubt its sexual. I read that some dogs do it just because they can, and they like it!
Its self rewarding I suppose. Going back to the OP, I would definitely discourage it in your puppy, but don't lose any sleep over it - loads of dogs do it, with very little rhyme or reason!! There is a school of thought that it is a way of exerting dominance over a pack leader or pack subordinate, as it is sometimes seen as non-sexual behaviour between dogs of the same sex, or a female mounting a male, etc etc. But then, the whole dominance theory is open to question isn't it.......
Susie :)