One interesting fact that I have observed. When I look around me at people I know who are happily married and those who are not, often there does not seem to be any correlation between how good someone’s marriage is and how “deserving” you would think the person is of a good marriage based on their middos and emotional health.
My impression is that being happily married is a skill, and there are certain things you have to know. Some people learn these things from observing their parents or other role models, some people learn them from a good Chosson or Kallah teacher, and some never learn them. I think there are people who don’t have good relationships with other people and/or may not be such good people, but they know what it takes to make sure they have a good relationship with their spouse.