I pretty much wave at all bikes, I even catch myself doing it in the truck. Growing up I would go on the road with my dad and my cousin and they waved at everyone especially when there was a girl in the car. I asked my cousin why they did this and he said it was interesting to see peoples responses, if you waved at a couple in a car the girl would usually wave and the guy would look at her with an accusing look as to say "Ok, who is that"?