The Uncanny Valley may be a deep, instinctual reaction: it steers humans, on an automatic level, away from humans who are dead, diseased, or deformed. In that way, the theory goes, the Uncanny Valley is a protection against sources of infection. Some psychologists believe that this effect is a major reason for racism and other forms of intolerance. To early hunter-gatherers, anything different from you was either food, a rival, or a predator. If something is recognizably human, we naturally assume they're either friend or foe. So something that's human but... different in some way creates cognitive dissonance. As said in The Hitchhiker's Guide to the Galaxy, there's something about people with insanity or mental problems, even if not clearly visible, something with the way they speak, move, and react that sets off warning bells in people's heads. All that said, humans also have the ability to desensitize themselves to the Uncanny Valley in cases where it would cause problems such as racism. If someone hangs out with other groups of people, then they are more likely to get used to them and the Uncanny Valley no longer affects them with that group. In the same vein, if you stare at a picture that freaks you out long enough (and it doesn't start to stare back), then you probably won't find the same picture scary anymore.
It might also explain the idea of the perfection of the human face. You try drawing a perfectly detailed face, and if there's one thing that's slightly out of place, or if the shadow's just wrong, or there's a small bump where there shouldn't be, the whole face doesn't just look wrong, it just feels wrong, despite it being quite a good drawing nonetheless. Of course, with pieces that are meant to be surreal, for example, surprise, surprise, the surrealist interpretations, you accept them. But with pieces that show forms of realism, like The Scream, the visage is... well, frankly, disturbing.note