Waving is a funny thing. I get waved at sometimes and ignored (if I wave) other times. I have three theories: 1) The rider who doesn't feel like waving is under some kind of pressure/stress (headwind, uphill, sun-in-the-eyes, worrying about traffic, fast downhill, etc.). The one who feels like waving is going in the opposite direction, not under stress/pressure. 2) An Instant Social Darwinist Impulse (ISDI) strikes both riders at the same time, they unconsciously size each other up (bike, kit, form, age, bag, etc.) and decide in 1/100th of a second whether the other is worth acknowledging or not. Either a wave from one or the other happens or it does not. 3) I'll never know what waving is all about, really.

I think about these things sometimes when I ride. I know- it's weird.

PS: If I get waved at, I always try to wave back, no matter what my ISDi urges me to do - its just polite.