on a nice summery day, i'll see hundreds, if not thousands, of other bike commuters along my route, but it's the big city; we're all doing our own thing and don't have time for other people. striking up a random conversation with another rider along my route would just be "odd" and uncomfortable for both parties. the anonymity of being one in a cast of thousands means you're on your own, which i greatly relish.
generally speaking, i don't enjoy riding with other people, even when i'm out on recreational weekend rides. other people just seem to get in the way.