Jonah Lehrer has a guess:
I've been recently been reading some interesting research on close, interpersonal relationships (much of it by Ellen Berscheid, at the University of Minnesota) and I'm mostly convinced that there's a fundamental mismatch between the emotional state we expect to feel for a potential spouse - we want to "fall wildly in love," experiencing that ecstatic stew of passion, desire, altruism, jealousy, etc - and the emotional state that actually determines a successful marriage over time.
Berscheid defines this more important emotion as "companionate love" or "the affection we feel for those with whom our lives are deeply intertwined." Jonathan Haidt, a social psychologist at the University of Virginia, compares this steady emotion which grows over time to its unsteady (but sexier and more cinematic) precursor: "If the metaphor for passionate love is fire, the metaphor for companionate love is vines growing, intertwining, and gradually binding two people together."