To George Weasley,
every mirror is the Mirror of Erised..

If you don't get it, when you look in the Mirror of Erised you see only your heart's deepest desire. George Weasley's deepest desire would be to see his identical twin brother Fred alive again. But even if he looked into a normal mirror he would think he's seeing Fred because the two of them were exactly identical.
It's weird - the twins were only ever hurt when they were apart. The first time they weren't together, George's ear got blown off. The second time they weren't together, Fred was killed.