At about 10, boys and girls start going into puberty. They start getting raging hormones, acne, changes in body size, etc. They will be wanted to find boyfriends/girlfriends about this age, because their minds will be all over the idea of sex by then. So, of course, one might get a little nervous about how they look. They'll wear makeup and too tight clothes or way too big clothes to make them seem "attractive". After that, it seems to just stick in peoples minds all through adulthood. Hope this helped.
Because it's the outside that they see, they can't always see the beauty that comes within, and honestly if anyone says that looks don't matter they are either lying or visually impaired. Looks do matter because that's what attracts you to someone when you first meet them it's their personality that keeps you coming.