EJGraff: Ah, why we marry today. Since the 19th Century, the main rule has been love. Once we can make our own livings we can also make our own beds. You're really asking two main questions, though: First, why do we want to marry emotionally -- to bond with one other person? People have done that throughout the ages for lots of honorable (and dishonorable) reasons. Second, why do we have the outer ceremony, the public registration? From my point of view, it's to adjudicate disputes. Given that the history of humanity is the history of disagreement, society always needs some way to decide who's being fair and who's being unfair, and to whom. Should my brother inherit, or should my beloved? Marriage is the marker (sorry to be repeating myself).
Runningincircles: Systemic control -- is that what you're saying?
EJGraff: Not control, no. Given that we bump up against thousands of strangers in our lives, marriage is a way to mark who belongs to whom -- whether I care about X enough to want her by my side if I'm ill, or whether that role falls to my mother or my daughter.
Kherrity: When was the first documented marriage?
EJGraff: Whoops! You know, they don't know that. Marriage exists in every recorded and discovered society.
Kherrity: Really? Do you think there has always been some form of marriage then?
EJGraff: Yes, apparently people have been finding bonds with each other for lots of reasons, forever. But the reasons that societies marry differ. For instance, one anthropologist was told that marriage was to create brothers-in-law ... so you'd have someone to go visit.