I'll stay true to the thread topic although I've experienced both sides of this.
Dumping and being dumped happen to the best of 'em if you live, and have a life, long enough. People are never as predictable as you might think they are.
The example I've chosen follows. "Jim" and I first connected after I saw a sports-related, non-sexual, classified ad he'd run in the local Gay paper. (That dates it right there, eh?) We became fast friends, burning up the phone lines and keeping the post office in business since we lived two hours apart. On the rare occasions when we got to connect in person it was GOOD TIMES. Aware that Jim had had battles with the bottle and was staying away from alcohol, I accommodated this without a second thought. When you so much as really like somebody it's no sacrifice. And we "really liked" each other.
Whether he was 100% Gay is debatable, and the point's moot, but Jim was in an LTR with a guy and we didn't take our friendship to bed. Everything went great, for about six months.
Then my phone rang on a bleak February afternoon. "Sup, dude?" "HEY, Jim, how's it goin'?" "Well, I'll tell you how it's goin'. I'm alone right now, I'm hammered, there are drugs and guns in the house, and I'm ready to use them."
For NINE SOLID HOURS I was held captive on that call. Jim told me the story of his life (just about his entire life), often breaking into sobs deeper and more aching than I'd ever heard. Whatever it was he'd fallen off the wagon with, he'd sip some more of from time to time. My mind raced as I tried to think of the right things to say; at least I knew enough to "just listen" when he started rambling. At long last his partner and some other people walked in the door, and he bade me good night after whispering profuse words of gratitude. Soon after that I got a note in the mail that went on with the heavy-duty "thank you's."
Right after I played Samaritans counselor, I massaged my "phone ear" and promptly called a friend who was well versed in such matters. He'd grown up with an alcoholic father and had a brother who'd also been lost to the sauce. The advice I was given was, "Dump him." I held out for giving the benefit of the doubt, but Jim ended up making it an easy decision. We had a few light, brief, happy conversations in the weeks that followed the call from hell. Then came a voice mail: "Yo yo yo, (MNOL)! And meow meow meow, (MNOL's cat)! Call me back today, man! But wait 'til afternoon 'cause I'm sooooooooooooo hung over."