I can not say confidently that I know the right answer on this question because I have no experience. Unfortunately, or fortunately, I am not married, but I have a person with whom I want to build a family in future. And I believe it will be happy for us. So I want marriage to be endless happiness.
The only things I can rely on, in answering this very question, are the marriage of my parents and my own hopes and plans for future marriage. When a stranger looks at my parents he will see a nice couple, when I do the same I see people who support each other and are happy together. Of course I see some gaps in their relationships, which I do not want to make myself in future. I also want my marriage to be happy. And I am ready to do everything to make it happy. The same I am waiting from my partner. All we need is love, mutual understanding and support, and our marriage will be endless happiness.
So relying on this two statements I can say that the marriage is a kind of test for both persons, the test which brings both bitter and sweet fruit, depends on people themselves. I consider it to be endless happiness, because I believe it is so.