Solo dancing does not exist: the dancer dances with the floor: add another dancer and you have a quartet: each dance with the other and each with the floor.

Just the pleasure of moving and the pleasure of using your body is, I think, maybe the main point. And the pleasure of dancing with somebody in an unplanned and spontaneous way, when you're free to invent and they're free to invent and you're neither one hampering the other - that's a very pleasant social form.