Albus Dumbledore: Back again, Harry? I see that you, like so many before you have discovered the delights of the Mirror of Erised. I trust by now you realize what it does. Let me give you a clue. The happiest man on earth would look into the mirror and see only himself… exactly as he is.
Harry Potter: So then, it shows us what we want. Whatever we want.
Albus Dumbledore: Yes. And no. It shows us nothing more or less… than the deepest and most desperate desires of our hearts. Now you, Harry, who have never known your family… you see them standing beside you. But remember this, Harry. This mirror gives us neither knowledge… or truth. Men have wasted away in front of it. Even gone mad. That is why tomorrow it will be moved to a new home. And I must ask you… not to go looking for it again. It does not do to dwell on dreams, Harry… and forget to live.
From Harry Potter and the Sorcerer’s Stone (2001)