It's hilarious to actually see the whole phenomenon play itself out - I mean, it only happens with bikes. When I'm in the car, fellow 'Stang drivers aren't waving at me: they're pulling up beside me in an attempt to race me, grind me to dust and laugh in my face at the next set of lights because their car is "better-than-my-car-so-I-get-all-the-chicks-suck-on-that-fast".
Car folks in general are always trying to one-up each other, whether you're in the "all-show-no-go" crowd or the performance crowd. It's when I ride my bikes I see all the happy-go-lucky bikers waving like love-struck tourists at each other (it's quite comical).
Yeah, I'm in the "I'll nod at you if I think you have a cool bike, you have a nice rack/butt, and/or I-want-to-steal-your-girlfriend" crowd.