I'll take a stab and use my sister as an example.

She is mid-30s and once again got involved with the reformed bad boy. Well bad boy turns out to be a really nice guy and she loses attraction.

So they have been living together for a couple of years. He wants marriage and she does when the relationship is going her way.

But when the relationship is rough going, her true feelings show through.

In essence she is settling and fools herself that he's they guy for her when times are good. Then the veneer cracks in bad times.

Truth be told, my impression is that if she were mid-20s, and not mid-30s this guy would be gone. But she feels too old to find someone more suitable and too selfish to let this guy go.