I believe the number one problem in marriage is and always has been selecting the wrong mate for one's self. This happens when you don't know who YOU are, what you want and need in a mate. You have to allow yourself to grow as an individual before you become one with someone else. What you think is a great catch at 21 may not be what you want at 30.
More people should take marriage more seriously on (the front end). By that I mean truly getting to know a propsective mate and finding out if you (naturally) agree on the things you value most. Opposites may attract in the short run but it's like that attracts like in the long run. Another problem arises when couples get married for the wrong reasons. A marriage based upon circumstances (baby, age, ultimatum...etc) rather than love is not likely to last. Awhile back I wrote a hub that goes into more detail about this.
One other thing to keep in mind, everyone has and is entitled to have "deal breakers". No one is "stuck" with anyone. If you marry someone and they become abusive, cheat, neglect you physically/emotionally or put you in financial ruins and so forth you are not under any obligation to stay with them. YOU are responsible for your own happiness. Yes it's true that when you are married it's less about YOU & I and more about US and WE; However "we" means that (both people are on the same page and want the same things for the marriage). If your mate wants to go left and you want to go right what is the point in staying together? A "team" moves in the (same direction) with the same goal in mind. Ultimately everyone is looking or someone who naturally agrees with them on the major things in life.
Dating is the phase where you do your "due diligence" to determine whether or not you want to have an exclusive relationship with someone or possibly marry them. Since most people put on their best face to "impress" others in a new relationship it takes time to truly get to know the "real person". Unfortunately too many people become emotionally invested or even engaged to be married during the "infatuation phase". Divorce is nothing more than one or both people concluding that a mistake was made.