Most Americans these days agree that couples should stay together only so long as both parties love each other. That should you fall deeply and irretrievably in love with someone else, you owe it to yourself to follow your heart. That you shouldn't remain in an unhappy union purely for the sake of the children. Marriage, the thinking goes, should entail joy and mutual self-fulfillment.
But not if movie characters act that way. We disapprove of other people who walk out on marriages because of "love" or "self-fulfillment" or the usual minor unhappinesses that occur in every marriage. Hollywood stacks the deck: you're leaving because he's a fascist heroin addict.
This is not so minor a matter as sorting out how fiction writers might most slyly manipulate their audiences' affections. Plenty of real relationships break up for no other reason than that one or both parties is "unhappy," or "in love" with someone else. Yet here in the world, we cannot stack our own decks. We can't rewrite the role of a partner with whom we've grown dissatisfied in such a way as to make our urge to flee more defensible. If your husband isn't, conveniently, a heroin addict, well--he simply isn't.
We may officially embrace the view that a partnership can justifiably be dissolved for reasons of "unhappiness" or "love." Yet we are not only the protagonists but the audiences of our own lives. Thus the same onlooker's standards of judgment apply to our private dramas as they do to entertainment. When we act trustingly on those ostensibly shared social precepts I began with, we're often shocked to discover that we don't sympathize with ourselves.