Originally Posted by Wikipedia's Uncanny Valley

The phenomenon can be explained by the notion that, if an entity is sufficiently non-humanlike, then the humanlike characteristics will tend to stand out and be noticed easily, generating empathy. On the other hand, if the entity is "almost human", then the non-human characteristics will be the ones that stand out, leading to a feeling of "strangeness" in the human viewer. In other words, a robot stuck inside the uncanny valley is no longer being judged by the standards of a robot doing a good job at pretending to be human, but is instead being judged by the standards of a human doing a terrible job at acting like a normal person.