This is not only a very personal subject but one that can be argued from couple to couple and person to person. The term happiness and marriage is really all about expectation.
What is a person's expectation of marriage itself? What is happiness? One aspect about two people coming together and marrying is that each person may have a very different view of what each of those two terms may mean.
How do you stay happily married without knowing what makes you happy?
The answer is that happiness is subjective and if you think that your spouse is supposed to make you happy than the marriage will most likely fail. When a person gets married they are expected to share with their spouse who they are and what they can bring to the marriage not what they can take from the marriage or how the other person can fill them up.
The marriage is about two people coming together and sharing with each other about who they are and what they can learn as a team. The couples that stay married are the ones that grow old together and learn from one another. They gain happiness from filling themselves up and then sharing that love with their spouse. Part of being a good partner is to always continue to work on oneself, to learn how to be positive and happy on your own.
When the marriage is not working the best possible plan is to get relationship counseling before leaving the marriage emotionally or cheating.
When the couple takes the time to work on the marriage and not brush it under the carpet then the marriage has a chance to survive. Understanding that when two people get married they are going to have times in the marriage when they feel happy about the marriage and other times when they feel distant from the marriage.
The secret is in the times of feeling distant is to stay and work on the marriage and have the knowledge that things can turn around in a marriage and can come back. Many times people just give up and try to find someone else to make them happy. If a person can go into the marriage and share themselves with a partner as a partner and not as a mother figure, father figure but as an adult (partnership) then when times do get tough they can work on the marriage together.
When the marriage gets to a point where a person can no longer stay in it and they think that they have done everything go to see a counselor before throwing in the towel in with or without a spouse, it is worth it too try before leaving the marriage then the person will know that they did everything that they could and leave with a clear mind.
Many marriage counselors can help the couple or individual to work on techniques to get the marriage back on track or to help the individual to work out some problems on their own.