That's a good question. I read this post a few days ago and had a bit of an issue with coming up with an answer. Putting aside the obvious answers such as abuse, the one though I kept coming back to is when you just don't want to be with your spouse anymore. You cannot see your future anymore, you aren't happy and it's not because you can't resolve certain issues. You don't feel the same, you've exhausted all possible answers and it's just not there anymore.