“Let me explain. The happiest man on earth would be able to use the Mirror of Erised like a normal mirror, that is, he would look into it and see himself exactly as he is. Does that help." 
Harry thought. Then he said slowly, "It shows us what we want… whatever we want…" 
"Yes and no,” said Dumbledore quietly. 
“It shows us nothing more or less than the deepest, most desperate desire of our hearts.”

- Harry Potter and the Sorcerer’s Stone