Agree with the dismissive replies. I do not know the true answer, but I, too, have often heard it had to do with symmetry (of course, an attractive symmetry) of features. A lot of people have one side of the face very different from the other side, and when observed from different points may or may not look attractive. Those I guess are the ones who" can't take a bad photo". Anthropologically speaking, symmetry, along with balance and coordination, produce subliminal clues that that person just might make a good breeding partner. So I guess having that person in your photographs guarantees interest?
Happy Holidays!