Forum Lurker's quite correct that it has a lot to do with the "unrequited" bit. A "lost love" from years ago stays forever young and attractive in our imaginations. In reality, they've probably gotten old, gray, bald, fat and ugly just like the rest of us. We romanticize about "what might have been" but, in reality, there's probably a damned good reason why it never happened in the first place.   

Bruce