There's a girl I know (I say "a girl" but she's really 32) who has just burned through her third cohabiting "committed" sexual relationship. She's bitter, of course. But from her conversations with me I can kind of tell that some of her expectations are unrealistic. Maybe the guy was a creep, but there are always two sides to these stories. Some people don't know how to "make up" after arguments or major disagreements, but there are cases where some sort of separation is inevitable. Call it "The Woody Allen Syndrome." Or maybe The Henry the Eighth Syndrome. There are plenty of men who are bad news.
Widely divergent differences of opinion, taste, lifestyle, politics, or religion (in mid-life) can also be potent triggers. Look at Jane Fonda and Ted Turner! And when it's not sex,politics, and religion, money can rear its ugly head in the oddest and most demented ways.
I never figured out the real story with Lady Diana and Charles Windsor though. Classic age difference and personality clash, maybe?
I love your comment regarding the narcissism of College mentality crashing into reality..... that is priceless!