I believe there is a major difference between dating and living together or being married. When a couple is dating they're still trying to make the most of their time together when they visit one another. This includes going out to dinners, movies, taking walks, making plans for upcoming events, and having passionate sex before having to sadly say good-bye.
However when couples live together or get married they tend to fall into a rut. More time together tends to lead to discovering (more differences) in each other. Both people believe they are "right" as to how things should be handled. Some people have a difficult time adjusting to sharing space. The more we have of anything the less valuable it becomes. A 20 year old does not place the same value on (time) in a life as someone who is in their 70s or 80s. Abundance of time together has a way of changing one's attitude towards their mate.
The couple never sat down to discuss what they expected from marriage. They did not create a "plan". They made an (assumption) that they wanted and expected the same things. When they were single their finances were separate and each person could spend his or her money anyway they chose. A marriage between a spender and a saver could lead to fights.
When there are major disagreements there is no getting in your car and driving back to your place. You have to try and resolve issues. Some people never adjust to being locked into a situation where leaving is a difficult option. When someone is unhappy they tend to take it out on the person who is closest to them.
People can't help but reveal their "authentic selves" when you live with them. It's seeing someone at their worst that will determine whether or not you can deal with them long-term. Rarely does one see the worst of someone while simply dating.