They are one of the few couples we see on Star Trek that aren't some kind of superhero couple (Book and Michael, Stamets and Culber, even Torres and lizard boy) and care about each other and the family they're raising together.
I think it's just that as the only married couple like that, they end up having to be the sole focus of all the "married couple" tropes for the writers; there weren't any other couples to write them about.
I forgot where i learned this, but O’brian was put in this position on an explorer ship to hide him, and let him recover from all the murdering he did in the war.
I think it was plague of grapes about why O’brian must suffer.