Over the course of human evolution, there were methods needed to detect if other potential mates had good genes that would produce healthy offspring. But back in the stone age, it was all about basically mating/having babies and raising the number of people in your clan. People needed immediate cues to determine how healthy the other person was (how good their genes are, so they can produce healthy offspring). You know that whole natural selection thing, people who tended to have preetty/symmetrical faces were of good genetic makeup, and were preferred. Also beneficial to evolution because if you can pick someone out immediately with good physical characteristics (such as facial symmetry), because back in the day, it was all about creating offspring and people just wanted physically healthy mates---and believe it or not facial characteristics are a good indicator of that (on average). There's a ton of experiments that show this correlation (but it's still a theory....and since psychology/study of behaviour is a "softer" science, it doesn't have the same quality of evidence harder sciences like chemistry or physics have). That's one theory I learned in psychology though ( basically nature programming us to respond to physical traits such as faces, because in the past it helped our ancestors find healthy mates, produce offpring, play the evolution game and pass down their genes,etc.) Though remember, it's just one theory among others.....