Disclaimer: I don't own Harry Potter.
He knew that some people might like to be a double agent, but they were people who weren't spies. They saw it from the outside, without the paranoia that distanced him from the rest of the world, without the overwhelming fear of discovery that forced him to stay in an invented persona twenty-four hours a day, without the terror aroused by a book being dropped half a corridor away. They thought it was daring and dashing, a great and dramatic lifestyle. They didn't realize that it meant you could never go down to the pub and have a few pints and a nice game of darts, because it would be out of character. They didn't realize the great and soul crushing alienation it caused, because it wasn't safe to let anybody close. For the most part, he hid his loneliness even from himself, and sublimated all of his desperate desires into his beloved potions. But then, in his tenth year of teaching, a profession he loathed and had joined unwillingly, he was asked to help set up the protections on the Mirror of Erised. He went late in the evening, after everyone else had done their part, so nobody noticed that he was missing until breakfast of the next day. He was found on his knees in front of the Mirror, tears silently streaming down his face. He stared at it even as they pulled him away, stared at an image that only he could see. An image of a little house somewhere, with someone to come home to, someone who loved him, standing in the doorway. After he was dragged back to his cold and empty rooms, all he could think was how much colder and how much emptier they seemed compared to the beautiful illusion of the Mirror.