After moving to where I live now, I've found that friends of mine who have coupled go AWOL until I am also coupled, after which we go out on 'couple dates' where the men talk and the women talk, but men don't talk with women and vice versa. Unfortunately, most of my friends were male so I spend the evening talking to his girlfriend who I have little to nothing in common with for virtually all of the evening. I have made new acquaintances that I really like this way, but it's not frequent.
My best friend from school, first grade through graduation, really hurt me by dumping me as a friend my first year of college because she was in a 'different stage.' She even wrote my mother a note explaining why she was distancing herself from me, explaining that it was because she had moved on to the next stage. She was living with a man and I was not seriously involved with someone, because I was going to school full time and working, so I didn't really have a chance to seriously date at that point. I did have a boyfriend all through high school, but I always spent at least one night of the weekend with her and talked pretty much every day the entire time I was dating him. Her mom would drop everything for the man in her life, so she probably picked up on that, but I still took it personally at the time.
One associates with those they have things in common with, especially in relation to where they are on life's path. While it's nice to have diversity in one's friends, sometimes you don't want to have a learning experience - you just want to talk to someone without giving a ton of background on the situation with someone who is at that same place. Unfortunately, I think some people take it too far and fire everyone who is not 'with them,' so to speak.