I'm no expert but I when I think about the times i was willing to walk away from our marriage and not work on it, it was for a couple of reasons:
1. I thought H would never change, never understand what I needed. 2. The resentment was too great on both sides and there was no way I could see us actually working through that to get to the other side. 3. It was just too hard. I was always under the impression that marriage should be hard but not THAT hard. I was wrong.
In the end though I never did leave so at some point I figured I should either accept the marriage as it was and make small changes in myself to view the marriage differently (the problem with this is that the resentment kept building so I'm not sure how long that plan would have lasted) or leave.
The problems felt so insurmountable that even looking at ONE of them made me so exhausted. It felt like leaving was the ONLY way to get away from the dysfunction, sickness, resentment and get what I really wanted which was a true partnership built on love, trust, dedication, health and support.
I can't speak for other WAS but I almost became one myself so I can speak a little bit about it.