Because you look like you're putting on an image. If you weren't self-conscious, you would look more natural. What really makes you look self-conscious/forced/etc are the additions you make to your photos, to take attention away from you and how you look; because if you're wearing sunglasses, you're sitting in the dark, and you're posing, people aren't going to be thinking about your facial features, let alone deciding whether or not they appeal to them. But if it was just you, with a minimal amount of stuff in the background, and possibly with you smiling, then people would actually be able to see you.
Think about this objectively: Who can decide whether or not you're attractive, if all those other factors are standing in the way? If you're in the dark, and wearing sunglasses, what good would plastic surgery do? No one would see the effects of the surgery, because you would be in the dark.
Edit: Here's another example. You know how on Facebook, some girls (and maybe a few guys, but more girls) only use webcam/Myspace photos as their profile pictures? That also shows self-consciousness -- because only when they have perfect control over how they look in pictures, are they comfortable showing those pictures to the world. (This is an especially good example when those girls only use webcam/Myspace photos that they have digitally edited!)