This may be obvious and I am sure that it is unfair to simplify it to this degree, but very roughly speaking, it seems there are 3 groups of A couples and the goal is to determine which group you belong to as soon as possible and at the least personal cost.
A) Couples that never should have married or even tried to become committed
B) Couples that once worked and had a chance, but went sour and cannot recover
C) Couples that worked, encountered a rough stretch, learned and grew, and belong together
Some folks may periodically shift from group to group, but they probably were really in that group in the first place and just did not realize it.
In retrospect, I was in category A) Couples that never should have married or even tried to become committed. It was doomed, but I enjoyed some aspects way too much to have passed it all up.