Haha my roommate does it to me, and like someone else said I wear my makeup for me, to feel good about myself. That means even on the days I'm not going any wear or seeing anyone I'm gonna wear it. It bothers my roommate she makes little rude remarks and whatnot. I think she thinks I'm vain. No I'm not, I hate my acne and I'd rather not look at it. I try not to let it get to me because people don't need to worry themselves bout what other people do, it's not their business to comment or share their opinion.
Because the problem is with the girl with the upturned nose, not the girl wearing the makeup. More often than not, it's jealousy. If I see a girl wearing a ton of makeup and it looks bad, I won't turn my nose up, I'll think what a shame she can't see that less is more.
Because they think the girl with the make up is doing it for boys, or to be vain. They don't realize that girls wear make up to make themselves look their best, which makes them feel good about themselves. No different than getting your hair done, or wearing nice clothes.