The monogamous relationship expectation from our society is another thing that can cause issues in relationships - you know the traditional relationship, the fun stuff dries up and one or both goes off on affairs or ladies (or men) of the night, gets caught then the court shenanigans start.
Maybe humans aren't designed to be monogamous, there's a term called ethical non-monogamy I see doing the rounds these days and maybe for good reason?
I find people interesting, I should have done psychology at school