I personally feel my own opinion of my physical appearance is very complex...
1. I'm gay so care what girls think of my appearance, and when looking at other girls, part of me thinks "I wish I had her hair/legs/face/boobs, etc" but another part thinks "She's hot", so...it's a bit weird I guess. Does that mean if I want to look like a girl and I think she's hot that I would think my own self is hot too? I also find it odd that some people say I look a bit like Grace, and that's great, and I do have the brown hair and blue(ish) eyes, but then...I find Grace hot and all that, but I don't find myself hot...so why is that? lol It's all too much for my tiny shrunken brain I think.
2. I still care what guys think of my appearance even though I'm gay...why is that? Who knows.
3. I feel much more confident when I know I look fairly good, as opposed to feeling very vulnerable and insecure if I feel I look bad (an extreme example of this is if I have no makeup on. I won't go outside, and if ever I have to, I wear sunglasses and don't talk to or look at anyone)
So basically, I personally think that the reason I find myself caring a lot about how others see me on the outside, is firstly because we're basically at the mercy of others' opinions. If someone doesn't like me, I can be screwed. If the boss doesn't like me, I lose my job. If the teacher doesn't like me, I have a hard time in school. If my friends don't like me, I am alone and not as happy. If nobody likes me, I suck. lol
So yeah, even though obviously not EVERYONE controls any aspects of my life, we're sort of used to caring what everyone else thinks. A lot of the time I don't care, I really don't, but when it matters I do. I have to or I'd have troubles getting what I want. I think how the whole thing with appearance happens is that most people will only see you for your exterior, since they simply don't get the chance to delve deeper unless they actually know you or talk to you for any length of time. So...you know. It's all we've got.
Secondly, I myself often feel I don't have a heap going on inside to make me appealing to others, so if I don't look good, I've got nothing.