First of all someone having cancer does not mean they make you unhappy. You may be sad that they are sick but their actions to do not cause you to be unhappy.
I agree that happiness is something you gain and lose. But if your marriage as a whole is less happy than unhappy, then you are not with the right person IMO.
If someone dreads coming home every day because he hates his wife, but she has forced him to go to church every week, volunteer at a local charity, and made him lose 30 pounds does that mean he should stay in the marriage?
To me the answer is no because there could be someone out there that can make him a better person and not make him unhappy at the same time.
I bolded the above to emphasize a major point. If someone was unhappy about those things, then that is when the "marriage" part needs to happen. The partnership needs to about communication and figuring out how both people coexist. What you are insinuating is that two individuals must act as separate people, with the sole purpose of making each other happy. That is not marriage, that is being lovers. That's why so many people get divorced, is they cannot separate the two.
Furthermore, if the above example is happening to a husband, the action shouldn't be divorce simply because he doesn't like the changes his wife wants him to make. How did we get to this place in society where this is the common thought process? As Whiskey mentioned, if you don't have unhappiness, then you aren't working hard enough on your marriage. The union has never, and will never, be about living a carefree lifestyle of unlimited happiness. It's always been about working with a life partner to become better human beings. That means you have to make changes, you have to work past unhappiness and you have to stick to it even when they make you unhappy. Because that is what you vowed to do. If your marriage is based simply on personal happiness, then you certainly shouldn't be married in a church. As one of major themes of a christian marriage is "to have and to hold from this day forward,
for better, for worse, for richer, for poorer, in sickness and in health". Nowhere does it say, "well... make sure you're happy or you're with the wrong person and you should find someone else". You should have figured that out before you made a vow to take on a lifelong mission of working through all situations, whether good or bad.