I just think that you are putting a lot of burden on this person that he may or may not deserve. He may think you are a dork, then again he may not at all and I think it rather unfair of you to assign negative thoughts to him just because of how he looks and the fact that he did not greet you.
More plays into this than a bicycle. Where you grew up east coast, west coast, south - large or small town will all influence the way you interact with others. I was not particularly raised to talk to strangers... I grew up and still live in a large city and people will think you are pretty strange and forward to greet everyone.... Call me uppity if you like, just remember that not everyone's social norms are the same as yours.
If I'm greeted while riding I will certainly try to return the greeting, but there are probably plenty of times that I'm sure someone thought uppity road rider, when I probably was just not paying any attention to them. I think its great to see people out on their bikes and I certainly prefer to see them riding than driving- no matter what they ride or why they ride. That will never make me outgoing, and I still don't feel the urge to wave at all of them...

