I especially like what dashingscorpio wrote and so would like to add to it:
I'm not married and have never been married but plan to one day. Over the years I've been witness to some amazing marital disasters and only two or three successful marriages (in which the couple are just as happy now as they were on their wedding day). In truth, I think that there are two very strong proponents that will challenge any and every marriage:
With money as a challenge in a marriage, it's usually a lack of it. Rarely do you find that married couples have a problem coping wih too much money, except perhaps in Hollywood which then leads to divorce any how. As dashingscorpion put it, you can't have couples clashing with one another, such as one being a spendthrift and the other a miser. Most likely, the miser is going to file for divorce as the money will soon run out. I had a friend who became addicted to gambling and so was divorced by his wife.
In another scenario, one partner wanted children and the other did not. But because the wife loved the husband so much, she agreed to have just one baby by him. In time, she didn't take on motherhood as she thought she could and so ran out on him leaving him and the baby.
I have other stories as well, but these two factors seem to be marriage killers. Perhaps my word isn't worth its salt, being that I lack the experience, but you be the judge! In parting, I'd like to say this, in the words of Sheryl Crow:
It's not having what you want
It's wanting what you've got