It's possible to just exhaust the value of a relationship.
And it's possible that Nuke is right and people are just a bunch of flaming assholes who need a mushroom cloud up their collective asses.
Look. If you go into something like a marriage expecting it to fail, then you have no one but yourself to blame when it does. Marriage takes commitment, because it isn't going to be hearts and flowers start to finish. There are times you are going to want to scream in frustration. There are times you are going to want to be cruel and "get even." There are times you probably are going to give in to those urges. Times she will as well. Each time, heck, each morning, you have to decide again that this relationship is more important than your pride. You both put the other before yourselves. Because together, you are stronger than you are separately. Because you have someone to share both your victories and your defeats with. Someone to lick your wounds before you go back out into the fight.
I've seen my grandparents in their old age. They made each other coffee, pulled out a chair for each other, each of them was putting the other first, and it mattered. Those kindnesses, little, insignificant though they might have seemed to an outsider, meant all the world to them. It made them happy. That's what I want. That's what I'm willing to fight and sacrifice for. Anyone who is willing to walk away just because, "Well, I'm just not feeling it anymore," shouldn't have gotten married in the first place, and has no business doing it again. They'll just be putting yet another through the wringer to sacrifice to their own selfish need. [/rant]