People stop dating each other in marriage. Both sides could be selfishly thinking "Damn. I'm pulling my weight in this marriage, why am I not happier? or "Why does he/she always do _?" or "He/she never does _."
Resentment builds up like a volcano.
The person you marry today will not be the same person in ten years. You can take that to the bank. The problem with marriage is just like any other LTR, interest level is in full swing. It goes up and down and rises with the remains of the day.
Look at her friends too, does she hang out with unhappily married or divorced women? Those friends influence her opinion of her own relationship.
From my experience, making up from a fight may seem like communication "paid off" like it "should": the art of talking and listening to each other. Creating further intimacy, right? Often, I've seen that the talk/fight creates a disconnected chasm of resentment causing two persons to drift apart.
Marriage is tough work.
What I see now is entitled people looking at their SMV constantly asking themselves, "f this, why should I stay if I got power, infinite options, and disposable income?" The ole grass is always greener on the other side adage.
Besides, society and church is much more accepting of divorce now. "It's okay hun, you deserve to find someone better." With no stigma or guilt, people feel free to bail if the marriage stops serving them.
