Don't take it personal. I see a few other cyclists out there during my commutes. Do I greet them or chit chat at the lights? No. And I don't consider myself rude, antisocial or unfriendly for not striking up conversation with complete strangers. I'm simply focused on the ride and scanning for traffic. Most of the time, when another cyclist greets me, I'm completely caught off guard and they're out of range by the time I can yell back to them anyway. The same goes for motorists or little kids who wave at me. If I don't respond, it's because I'm really not expecting it.

But if someone is sitting right next to you in traffic and they don't acknowledge a friendly greeting, then I'd just call it like it is.