Most couples just can't be happy

As far as I can tell from fiction (and even some nonfiction!) from prior centuries, having strong societal* expectations of permanent marriage doesn't do much for happiness, but does do a lot to normalise infidelity.

* it also seems evident that what we call "societal expectations" were most strongly held by certain layers of their societies, and other layers were much more willing to shrug and say "life goes on", "you never can tell", etc.

Lagniappe: https://www.youtube.com/watch?v=b5AJUkDcLIU (when I was much younger I thought this was a sad —consider the tune reprised— ending; although the cinematography has not changed I have and I now consider it a happy —if not entirely American— ending)