1) Very few people have perfectly symmetrical faces, and while I personally find quirks and asymmetry really attractive, scientists tend to quantify beauty in terms of perfect symmetry. "Look at this face, rate it on a scale of 1 to 10. Oh, look, the highest scores had the most symmetry! LOOK, WE MAPPED IT OUT WITH GEOMETRY."
2) Many attractive people do not think they are attractive. We tend to assume they're modest, deeply insecure, or fishing for compliments.
3) Furthermore, a lot of people specifically think they look bad in photographs.
4) Faces can look really, really weird if you reverse the image. There was a site that did this with celebrity faces, and some of the pictures were, truly, pretty strange-looking. Maybe this is only because it's the reverse of what we're used to seeing; maybe--although I'm not sure how this is logical, but I'll throw it out here anyway--maybe something is attractive when it's flipped one way and legitimately unattractive when it's flipped the other.
5) I'm guessing that this is due to natural asymmetry, which most people have to at least some degree--the more uneven the features, the more drastic the difference. A perfectly symmetrical face would look hardly any different, if at all, but few people have perfect symmetry.
6) What you see in the mirror, in any reflection, is the reverse image of your face. In fact, the only time you're really going to see yourself the way other people see you is... in a photograph.
7) So the reverse image is what you usually see. The reverse image is "me." And therefore, people don't like photographs because photographs aren't "me."
8) People don't like the way they look because they only ever see the reverse image. If flipping an image results in something that is genuinely unattractive (rather than just "not looking like what we're used to seeing," because people are used to their own reverse image), people may only know the "unattractive" image in the mirror as "me."
9) You may actually be really, really handsome/pretty and not know it.
10) But if you're vain--the mirror-kissing type, as it were? You may not be as hot as you think you are.
Food for thought?
(Don't feel bad. I look okay in mirrors and terrible in photographs.)