It seems that the brain, in specific situations, literally gets bored and starts scaring you. The easiest way to prove this is to perform the simple experiment of looking steadily into a mirror, for a few minutes at a time. Soon, you're very likely to see a monster. That monster is a combination of your face and your brain. Does that make it better or worse?
There are a lot of creepy situations that start happening when you look in the mirror. Low light and a fearful mood certainly help, but the primary reason why people have so many mirror related freak-outs, and why it's become such a big game at slumber parties, is straight biology. The brain doesn't have the energy or the processing power to notice everything all the time. Sitting at your computer now, you're probably unaware of the feel of the seat under you, your clothes against your skin, and any lingering smells you might have noticed (no judgement) when you walked into the room. Your mind mostly tunes them out. But the sense that most of us rely on almost all the time, sight, has also been narrowed down. You are probably unaware of anything outside of the range of the computer screen, and you probably haven't noticed minor changes to that. That is why most updates on computers come with a sound or a blinking light.
The brain, when faced with a lot of stimulation, only some of which is considered relevant, will tune out the non-relevant parts, filling in what it can from the general area. It's a little like how the blind spot works, except this is a dynamic process. The brain will zoom in on a desired area, and the rest of the space will fade away. This is called the Troxler Effect, or Troxler Fading. It was discovered way back in 1804 by Ignaz Troxler, a physician and philosopher. Take a look at the circle to the left. Focus on the red dot at the middle. After less than thirty seconds, the circle should just fade away. The mind then fills in the area where the circle used to be with the white that surrounds it. It's worth doing an image search on Troxler Effect, since there are a ton of illusions with it on the internet. There are whole paintings that fade away. There are moving objects that disappear with enough focus. You can spend a happy twenty minutes observing your brain erase the world.
A less happy ten minutes would be spent staring in a mirror. A paper in Perception outlines an experiment in which people were asked to stare into a mirror, in low light, for ten minutes. They do not sound like a fun ten minutes, according to the report.
The descriptions differed greatly across individuals and included: (a) huge deformations of one's own face (reported by 66% of the fifty participants); (b) a parent's face with traits changed (18%), of whom 8% were still alive and 10% were deceased; (c) an unknown person (28%); (d) an archetypal face, such as that of an old woman, a child, or a portrait of an ancestor (28%); (e) an animal face such as that of a cat, pig, or lion (18%); (f ) fantastical and monstrous beings (48%).
The Troxler Effect fades out features that a person isn't directly staring at. Those features are filled in with what's around them. It works on a white background, but a face looks horrifying when, for example, a slice of forehead and cheek are subbed in for an eye. Plus, the effect doesn't stay in one place. The Troxler Effect wanders over the entire face, distorting it massively. The person then often instinctively turns those distortions into things that they can actually recognize, even if it scares them. This is how mirror monsters, like Bloody Mary, develop. The brain gets tired of processing the mundane and, accidentally, cooks up a monster to entertain us.