I had a really really good friend from freshman year in college up until my senior year. She was dating someone when I was single or vice versa. I never really thought of her as much more than a close friend (we would see each other at least 2-3 times a day). It snapped senior year when she tried to get me to cheat on my GF with her, I said I couldn't and she left my apartment never to talk to me every again.
Kinda sucked to lose my closest friend at the time when I was going to a ridiculously stressful part of my life (working + studying for MCAT + classes).
The worst part is that she went nuts after that, slept with like 6-7 different guys in a few months, got a bunch of tats, got a few STDs. Oh well.