I ride, I wave....if someone doesn't wave back, I assume he didn't see me or didn't feel like it was safe enough to take a hand off the handlebars.
The vast majority are friendly enough to wave back though. If someone is snobbish when I approach them, five minutes later, it's all good. I've been around, worked in too many states to name, etc, etc, etc and can always make one connection to that person in one way or another. Sometimes, it starts with one, simple, compliment ie "nice bike."