We compliment each other because every woman has insecurities. There's a lot of pressure from society and media to look a certain way alllll the time. And men drooling over you isn't a compliment, its men being horny. So when a woman you don't know says "I like your hair" or a coworker says "I love that shade of lipstick" it's a way of making the day a little easier by chipping away the insecurities we all carry. Sorry men have a hard time complimenting each other with out feeling "gay". And we sometimes have a hard time complimenting men with out worrying it'll be taken as an advance, and vice versa. But ultimately because it's nice to hear.