From a very good article on marriage:
With all this talk of institutions, norms, and obligations, you might be wondering: “And where does love fit into the picture?” Thomas Cranmer provided the best response in The Book of Common Prayer, when he directs marrying couples to promise to love and care for each other “for better for worse, for richer for poorer, in sickness and in health, until death do us part.” In this vow, the marriage is bigger than the couple—it’s an institution with its own norms and obligations. This elevation of marriage to the status of institution is not a belittling of human love but a tribute to its peculiar power and goodness. Cranmer’s lofty marriage vow upholds what the Western legal tradition has commonly recognized as an equally lofty good of marriage: a union of mutual support. In other words, marriage is friendship.
But not just any kind of friendship—in particular, the institutional model seeks to create what Aristotle called “the best kind of friendship.” Aristotle argued that there are three kinds of friendship: for the sake of pleasure, utility, or good. The highest kind of friendship, for good, is one in which each wishes the other well for his own sake. In this kind of friendship, friends are bound together not because they seek to use the other person, whether it’s for pleasure, a better social position, or economic benefit—no, they are instead bound together by virtue. Because of their good character, they love each other not because of any “benefits” but because of the intrinsic good of the other person. Obviously, then, this kind of friendship is possible only between good persons. As Aristotle explained, “mutual love involves choice and choice springs from a state of character.”