For me it was seeing my wife giddy like a crushing teen over another person. These emotional affairs happened a few times around 5-10 years of marriage. I'd see the signs, we'd have it out, she'd stop, get depressed... I tolerated it because these were with other women. These were not just close friendships, but intense longing for something I couldn't give.
Then for a long while things got better, more sex, kids getting older.
At almost 20 years there it was again. We went to a party at a co-worker's place and I saw the giddiness, the following around the guy's wife like a puppy, and I felt sick. It was different now. I could see the cycle would continue over and over. I didn't want this to be my life. Our sex life died and I finally got her to admit she is gay.
Am I happier? Yes and no. I'm glad to be out of that cycle, and away from other overbearing things about her. A couple of the kids didn't take it well. It's a lot tougher financially maintaining two households. But my problems are now my own and I deal with things the way I see fit. I dated someone and it was fun for a while, but neither of us wanted it to be permanent. I'd like to marry again at some point, I think, but I'm wary.