Mirrors are gateways. And, like most gates, they have guardians. People call them "reflections".
Oh, don't worry, they're not evil. They're actually trying to help you.
"Help how?" you may ask. Well, there are some ways to go "through a mirror", but things are slightly different there. Everything seems and looks the same, but it isn't. Probably you won't notice it, because usually no one notices it, but there is a difference. The problem is that the differences sum up. Every time someone walks in those gateways, something changes.
Well, water acts like a mirror, right? So, unwillingly, swimming in the sea, you just changed something. The same thing goes for polished metal. Every time you open a door that has a polished handle you actually pass through a gateway.
Now, think about it. People are constantly changing the world, but what if things get worse? What if things become so bad that no one has any hope left, and just want to save people from enduring the same things they endure? Then they become reflections, that try to stop people from suffering their fate.
You probably think that this can't be true, because you know that things can't get worse.
Well, if you think that, maybe you're a reflection.