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.