Cato Unbound has an interesting series of essays about the future of marriage, by historian Stephanie Coontz, and the economic factors driving the evolution by Betsey Stevenson and Justin Wolfers. (Wolfers has a related post at the Freakonomics blog as well.) Its no surprise, considering the Cato Institute's involvement, that a great deal of skepticism is directed at the State's involvement in the institution, i.e. the bigoted American obsession with "protecting" marriage. But the history here is interesting in its own right, and absolutely essential if you want to have any understanding of what's going in pretty much every novel written before World War I. Novels in their heyday dramatized the shift from more or less arranged marriages to love matches, offering all sorts of prescriptions for what love really is and proving a host of psychological and sociological defenses of the change. If I hadn't sold all my lit-crit books when I moved to New York, I could probably even cite the appropriate scholar on this, but it has been argued that the commercial novel pretty much emerges from that shift; it's a salient dramatic set piece that everyone in Western culture could relate to. (Maybe I need to go reread Denis de Rougemont.)
As Coontz explains, it made sense for state institutions to involve themselves in marriage when the practice was fundamentally a matter of broader family alliances than the preference of the betrothed individuals.
Because of marriage’s vital economic and political functions, few societies in history believed that individuals should freely choose their own marriage partners, especially on such fragile grounds as love. Indeed, for millennia, marriage was much more about regulating economic, political, and gender hierarchies than nourishing the well-being of adults and their children. Until the late 18th century, parents took for granted their right to arrange their children’s marriages and even, in many regions, to dissolve a marriage made without their permission. In Anglo-American law, a child born outside an approved marriage was a “fillius nullius” - a child of no one, entitled to nothing. In fact, through most of history, the precondition for maintaining a strong institution of marriage was the existence of an equally strong institution of illegitimacy, which denied such children any claim on their families.
In short, marriage was a system that supplied a legal framework for inheritances (and maintain gender roles, but that's other story). That changed with the rising discourse of happiness, and marriage as permanent property arrangements was replaced with marriage as a public recognition of companionate love. A great deal contributed to orchestrating this shift; it correlates with the rising middle class and relaxing of the rigidity of the class system. But whatever the causes, it has left us with an institution patently rife with contradictions.
The same things that have made so many modern marriages more intimate, fair, and protective have simultaneously made marriage itself more optional and more contingent on successful negotiation. They have also made marriage seem less bearable when it doesn’t live up to its potential. The forces that have strengthened marriage as a personal relationship between freely-consenting adults have weakened marriage as a regulatory social institution.
Stevenson and Wolfers put this in a more explicitly economic framework: when we say couples are together because they love each other, what we mean is that share "consumption complementarities" -- they share similar tastes about all the good stuff consumer society brings us.
Most things in life are simply better shared with another person: this ranges from the simple pleasures such as enjoying a movie or a hobby together, to shared social ties such as attending the same church, and finally, to the joint project of bringing up children.
No longer preoccupied with productive concerns -- with capital formation and division of labor in producing a household -- couples now can focus on consumption; this Stevenson and Wolfers call "hedonic marriage." And this change seems indisuptable. But as our tastes are subject to change, complementary tastes seems a flimsy pretense to enter into contract with someone that lasts until death does you part. Though I am sure this happens all the time, it's probably not necessary to marry the people you might like to, say, play bingo with. This shift likely puts implicit expectations to share all tastes on a couple, and probably subjects marriages to all sorts of undue pressure. Making the marriage commitment can in some ways become a kind of pledge to never change your tastes fundamentally, to become, to a degree, static. Obviously this doesn't happen to all wed couples; perhaps some share a taste for curiosity.