A pernicious mechanism that compels one to want one's ex more than one would if that ex were a complete stranger, platonic friend, or active love interest. While some may attempt to explain ex appeal in terms of nostalgia or fear of change, the phenomenon remains an anomaly, as it could not at any moment in the history of the human race have promoted survival and so should have been squandered by selective pressures hundreds of thousands of years ago.
Brandon: Why are you still hollering at Alexandra? She got mad fat.
Reynold: She's got ex appeal.