I honestly believe that many of my relationships ended because of drugs and alcohol. Either my use, their use or both. It was little things that added up to so much. Tempers that flared too quickly, money that was spent unwisely, unkind words that were said in a drunken confession.
I dated someone a couple of years ago that was really big into booze and prescription drugs. What killed the relationship for me was A. at his place one night with my two dogs, he cooked a nice dinner, gave the pups bones, we watched movies and he then passed out from drinking and drugging. I was irritated so I left (no, I wasn't drunk), two dogs in tow. The next day, he didn't even remember that we'd been there. and then B. he was so wasted one night that, with my little dog in his lap he jumped out of his chair, causing the dog to go flying to the floor. NOPE. Really great, wonderful man. But... not going to risk that. I couldn't leave him alone wih my pooches.
I've ruined my relationships since college by being a very difficult person to love because of booze. Moody, argumentative, angry, selfish... It's never enough for me. Even if I've ended the relationship, I've been left wondering why they didn't. Sure there must be other issues there but, alcohol brings out the edge. And when you're drunk 25% of the time (age 25) up to 75% of the time (age 30) it's not pretty.