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What did one of them find out about their partner that was so shocking, so disappointing, so repulsive that they just had to call it a day and walk out?
Sometimes you find out, sometimes you don't. Sometimes the aggrieved spouse is only too keen to dish the dirt on their other half and demolish their glossy image. Sometimes they're just too embarrassed to explain, too stunned by their own inability to see their partner's weaknesses before they tied the knot.
They may discover that Mr Right is a chronic womaniser, an alcoholic, a bully, or a closet gay. Ms Perfect may turn out to be a spendthrift, a neat freak, a self-pitying whinger, or a kleptomaniac.
Of course we all uncover unsavoury characteristics in our loved ones sooner or later. Nobody can keep up a phoney facade of rectitude forever, particularly if you're living with someone day in and day out. Eventually the cleverly-constructed mask will slip.
Most of these peculiarities we can adjust to as a minor aspect of our partner's rich and varied identity. But sometimes something emerges that is so alien, so extreme that we simply can't stomach it. Then the relationship dies overnight.
I scratch my head at the self-delusion of individuals who're well aware of some serious personal failing but get involved with someone else in the hope either that their unattractive defect will stay miraculously hidden or that the other person will be infinitely understanding and indulgent. Not surprisingly, their wishful thinking generally ends in tears, and the house of cards collapses in a startling heap.