I like to ride alone. I go as fast or as slow as I feel, stop where and when I want, and enjoy the ride 100% more than I would on someone else's schedule.

I'll ride with friends occasionally, usually to events, movies, etc. where we are all going to the same place. We're all casual riders (none of us race or really care about "training rides") so it works well.

I have no problems with any other, for lack of a better term, bicycle sub-groups. I don't get the whole head down, grunting, gotta try and drop everyone else type of riding, but some people seem to enjoy it. At least they say they do, although I never see them smiling while they're riding, so they may want to tell their faces...