I'm not a friendly guy but I act friendly to strangers and acquaintances as I pass them. It seems to make life easier when people are smiling and nodding at me. So I nod and smile. The good vibes energize me much more than the energy spent waving, so it makes me faster and more in the zone.

Its just an act though, superficial if not downright phoney. So I understand why some people don't wave back. They are rude, pompous jerks, yes, but so am I once you get to know me!