It is a perfect mirror in that it reflects back to us the things we're bothered by when we see them in others because we are ourselves guilty of the same. I wrote my take on the idea here: https://medium.com/illumination-curated/the-universal-mirror-and-how-it-can-help-you-live-a-better-life-d0de9b7b5e66?sk=5297c1c6c5260cae568b3b4084c690eb