I did my first organized ride last weekend and noticed the same thing. At first it sorta hurt my feelings and then it confused me, people didn't look like they were having as much fun as I was??? If it's not fun, why do it? Then I just assumed since I was a goofy newbie that was all excited and waving and saying hello that I'd committed some huge cycling faux pas.

The same thing comes up constantly on motorcycling sites (only there it's "why don't HD riders wave to non-HD riders?") and I just assumed it was a guy/cool/macho thing.