You can't compare short high school / college relationships to a long term marriage. After 2 or 3 DECADES of mutual commitment to each other, we have long proven to be compatible with our spouses. Not so with your school flings.
The fact that the M was good for so long does not escape the MLC'er, regardless of what they say. I'd even say they DETEST this fact... which is why they have to resort to writing history.
Did you hear many complaints prior to crisis? Yeah, me neither.
Ending a decent long term M is not easy, even for those who want to convince themselves it's what they need to do to find what they are searching for. This is why so many of them procrastinate on the D. They really aren't so sure it's the answer.
It's also why if we can outlast their crisis, we likely will have an even better marriage.
M: A really long time. Crisis: 5 years. She's still worth it.
Life is never made unbearable by circumstances, but only by lack of meaning and purpose. -Viktor Frankl