It must withstand change, aging, loss of youth, loss of beauty, loss of youthful hopes, and an expectable lifetime full of disappointment. But if that promise is made to hold, one is never alone, never in despair, never lost in the universe. One always has a home. Marriage is not about being in love. It is about the agreement to love one another.
Love is an active, transitive verb.
It is nice when married people are in love with one another, but if they are loving enough to one another, that magic may catch fire again. There is a relationship between love and marriage, but it is oblique. Marriage is not supposed to make you happy. It is supposed to make you married, and once you are safely and totally married then you have a structure of security and support from which you are free to make yourself happy, rather than wasting your adulthood looking for a structure. The state of marriage can make people happier even if the particular partner is a disappointment or an irritant.