This is pretty funny... I'm wondering though, do you think the main reason it's funny is because it's something we all do, in private, in front of a mirror, and now we get to be on the other side of this kid's mirror watching something very private? Getting to participate with him in his joy away from the prying eyes of the world, perhaps is a better way to say it, and thereby relive our own joy from when we last did this in the bathroom (you know you've done something like this at least once).
Meh... maybe it's just a mistake to try to intellectualize humor at all.
Thanks for the post.