Needing Time Alone
This is probably the largest problem that people have, the result of one or both individuals realizing that everything that they do is going to be with this other person. One of them may feel that they are obligated to spend all of their free time with their spouse, causing them to distance themselves. Others may have been used to spending time with friends, something that typically diminishes the moment that you get married. This has actually caused many people to get a divorce, simply because they were not able to communicate these problems and resolve these differences in an amicable way.
My Way Or The Highway
For people that are used to getting their way, or not having to discuss decisions with another person, and marriage is not the best place to be. A marriage is a union of two people, something that requires people to talk about decisions they are making, and if they are not able to realize this, they should probably have remained single.