What is it about mirrors that makes them so effective at scaring the bejeesus out of us? Visually, it is jarring: the camera angle is usually such that an unexpected figure in the mirror distorts our sense of reality, for we only see one person in front of the camera. In effect, for a brief moment, the monster is us, the audience, as well as the character, by our adoption of her point of view. In the context of horror films, surprises are often of the unpleasant kind. We are conditioned to fear the unexpected (part of the great unknown), so that even if the monster wears a human face, for a brief moment, we experience that jolt of fear. While mirrors may be used cinematically to express a character's fear from within - and thereby ours - they are effective emotional device that banks on our fight or flight instinct triggered by the confrontation of an unexpected physical event as well.
Furthermore, mirrors themselves are quite strange - the ability to recognize one's self in the mirror has been linked to consciousness / self awareness. You, as human beings, understand that the reflection in the mirror is actually yours and are not terribly confused by the lack of an actual person in the mirror. While some people may claim consciousness to be the sole province of humans, researchers have set out to test for presence of self awareness in other animals. Elephants, for example, have been found to be able to indicate that they recognize when their 'face' is painted (Plotnik, de Waal, & Reiss, 2006). Luckily for them, there isn't any elephant horror film to speak of. The fact that we are self aware and thereby understand what to expect of mirrors is exploited in horror films, as filmmakers subvert our expectations of normal mirror behaviour, much to our delight (or horror?). So the next time you jump out of your seat when that mirrored medicine cabinet is closed and a monster reflection appears on it, don't be ashamed of your manipulated state of fear. You're only human.