In a way, I guess I understand not waving at everyone you see. Some days on my commute home, I've had a bad day at work, and I just want to forget that people even exist. I just don't feel very social. Cycling is my escape on these types of days. Sure, if someone waves and I see them, I'll wave back; but it's at those times that I just want to be left alone. It's no intentional slight, it's just the way I feel.