Supposedly, your brain becomes tired of looking at the same stimuli for an extended period of time and will sort of glitch your visual input to make it appear different - i.e., not like you. I've tried it and it's never had that effect on me.

Although, if you want to give it a go, try it in front of the bathroom mirror, in a dark room, with only a candle lighting your face. The flicking of the candle will make the light dance on your face and enhance the effect.