We are equipped comparatively well to go on with a partner for a few years, probably a maximum of the famous seven. It's about an age when the first-born children could follow a herd independently. I don't think mankind is equipped emotionally to last in love relationships for more than a few years, certainly less than ten. When we fall in love, we just see all the good sites of a partner or prospective partner. If a relationship proceeds to this point, and beyond, it's just fading down.
Life can be so depressing, day in, day out.
There is nothing left to talk about. And then it's time to die. Each of the two partners will contemplate what went wrong. If they lack understanding of life itself, they may ultimately think that it all comes down to the wrong choice of partner. They'll remember the other options they had in life, those that they bypassed because they decided for the one they're with.