Esther Inglis-Arkell in io9:
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. 
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.
More here.