While there is no end to the problems that can cause marital breakups, in reality they often boil down to one simple factor - selfishness. One spouse acts selfishly, or more likely, both spouses are very selfish, and this leads to friction, thoughtlessness, poor communication, and even downright abuse.
Think about how this is true today. Don't most people just want to do what they want, when they want, how they want, without any regard for the consequences or for those around them? The husband wants to play poker with the guys and doesn't think about whether or not his wife hasn't seen him in days and needs help with the children. The wife wants to buy that fabulous new outfit or piece of furniture she sees without any regard to the balance on their credit cards. Both get an idea in their head about what is the "right" way to discipline or train the children, and neither is willing to compromise or budge an inch.
Selfishness also manifests itself, not just in what people do, but often in what they refuse to do. A wife's or husband's mother may be insulting to the spouse, but she or he refuses to stand up to the parent because it's just too unpleasant or because it makes them feel good to know that they're still someone's child - without considering how this makes their spouse feel. A husband or wife refuses to set proper boundaries with their children from a previous marriage because they don't want to be the bad guy with their children, regardless of how this is affecting their marriage.
Even in little things, this selfishness manifests itself in what spouse's do or don't do. A husband doesn't want to help with the housework, so he just doesn't. A wife doesn't want to balance the checkbook, so she just doesn't. Toilet seats get left up, toothpaste tubes are left out, overdraft fees are racked up, the house goes without needed repairs … and so on. These seemingly small things can often add up to that proverbial "straw that broke the camel's back," with not just any one of them being the reasons for a marriage to be on the brink of divorce, but many of them together making a couple just grate on each other's nerves.