It's in the Symposium. Humans started off as androgynous souls with four arms, four legs and two faces on a single head. Zeus felt they were too powerful, so cut them in half. So now we spend our lives searching for our other half, to complete our self. According to Aristophanes.
The first thing that springs to mind is C. S. Lewis's famous argument (in The Allegory of Love) that the medieval French troubadours effectively invented romantic love. Fin amour is initially purely a literary fiction. Then people start turning art into life, much to the religious establishment's chagrin, since the fiction is founded on adultery, (marriage being a political-financial deal, essentially). Then, I suppose, marriage has to take on these trendy ideas of romantic idealisation in order to be appealing again. Something of the shift has happened between Chaucer and Shakespeare, but it's interesting that although Shakespeare's happy endings are marriages, I couldn't think of a single play in which we have a happily married couple represented.* That comes later, I think.Found Lewis saying elsewhere that 'the conversion of courtly love into romantic monogamous love was...largely the work of English, and even of Puritan, poets'
One would probably now want to ask where the troubadours got their new ideas about love from. I think the most interesting theory is that the crusading princes picked it up from the Arab poets.
http://en.wikipedia.org/wiki/Courtly_love#Andalusian_and_Islamic_influence
*In fact, I can't think of much later literature or art on this...