This is an interesting question, and I wonder the same thing myself. I think it comes down to a couple of things.
First, when I wear makeup, people notice and say something nice, usually. I think it's because I look a little more put together than usual, because usually I don't wear makeup, but it's easy to assume that I just look better, prettier, whatever. So there's positive reinforcement there.
Second, it's a confidence thing. Almost like a superstition, really. I feel better about interacting with people, particularly people I don't know, when I have a little mascara on. I don't have to do up my whole face, but a little something here or there to accentuate a feature I like helps me to feel like contributing more to a conversation or situation. I am aware that this is silly, but I think there are worse things to use to perk your esteem up a bit.
I think that girls that wear a lot of make up have a self esteem issue. They don't think they look pretty enough in the first place so they have to make up for what they think they lack by putting make up on. A little make up is ok but I personally only wear make up every blue moon and even this its very subtle. I really don't get why girls want to wear make up to make them selves pretty. I think its all about natural beauty. I don't think I look that good but I'm not going to put some stupid fake sh*t on my face to make myself look better. If a guy notices me I want him to notice me for what I really look like not some mask I put on because sooner or later that masks going to come off. Not every girl wears make up for guys for like I said I think it goes back to self esteem issues weather they want to admit it or not.
Ya I hate it when girls wear a whole lot of eye make-up to supposedly make themselves look better - in my opinion it actually usually makes them look worse. But wearing some make-up, styling the hair a little [making sure it's smooth at least], and wearing neat, possibly fashionable clothing does make girls a whole lot more attractive than doing nothing.
When it comes to looks balance is key, though some girls think that the more stuff they put on themselves [make-up, fake tan, hair products and extensions, etc.] the better because they think that they're naturally ugly...but in reality they're usually naturally pretty and all that stuff actually makes them look worse. Sorry if I offended anyone but it's true and I bet many guys would agree.
It might seem like they're trying too hard to you, but some girls actually enjoy taking care of themselves as they like looking good. And, believe it or not, not all girls put on make up for guys. Oh noes, the world as you knew it is coming to an end, innit?
Seriously, some guys just flatter themselves too much.
Because guys make them feel pressure to be perfect.
Honestly, do YOU think she looks better with make-up/mascara? If not, tell some girl that you think she's beautiful just the way she is and that she doesn't have to go through the trouble. If you can't manage that, then obviously it does give an effect. Granted, some girls overdo it.
Girls: everybody is different, don't try to fight it.
Some girls think makeup is fun. I'm one of them. I like wearing it. It isn't a confidence issue, I have no problem leaving the house sans makeup, I just look at it as an art form and it's fun. Many girls enjoy putting an effort because it makes them feel good. Most guys don't know or care whether you are wearing blue or black eyeliner but we do so we care.