It cheers me up no end when a lycra-clad person on a £2000 bicycle nods or says 'Hi' to me as they pass, even though I'm riding a £60 second-hand bike and wearing jeans and my (fake) Doc Martens. Maybe I'm too easily pleased. I always reciprocate, of course.
As a species we evolved as part of a tribe or small community where individuals would know each other, and therefore would normally acknowledge one other when meeting. Cycling has a tribal aspect to it, hence the tendency of (some) riders to nod, say 'Hi', etc.
Steve