Did you ride an elevator lately? Was there a mirror on the wall of the elevator? Why is that?

How did you feel when you last rode an elevator that had no mirror and no glass walls? 

The main reason we commonly have mirrors in our elevators is to solve a common problem. Riding in an elevator is boring. We could put in a picture. But a picture would also get boring after a while, if you rode the same elevator at least twice a day. Cheap, but not much of an improvement.

We could put in a TV screen, as some do. But it makes the elevator more expensive. And it also requires a lot of maintenance. We need to put some content on the screen, to make it not too repetitive. So either there's a person whose responsibility is to renew the content once in a while, or a third-party company that does it for us. We'll also have to handle different problems that may occur with screen hardware and the software behind it. Seeing the "Blue Screen of Death" is amusing, of course, but only mildly. 

Some architects even go for putting elevator shafts on the building exterior, and making part of the walls transparent. This may provide some exciting views. But this solution also requires maintenance (dirty windows don't make for the best view), and a lot of architectural planning.

So, we put in a mirror. You get to watch an attractive person even if you ride alone. Some studies indicate that we find ourselves more attractive than we are, anyway. Maybe you get a chance to review your appearances one last time before that important meeting. Mirrors visually expand the visual space and make the entire trip less claustrophobic, or less awkward, if it's the start of a day and the elevator is really crowded.