Dumbledore: Harry, do you know why Professor Quirrell couldn't bear to have you touch him? It was because of your mother. She sacrificed herself for you, and that kind of act leaves a mark. No, no. This kind of mark cannot be seen. It lives in your very skin.
Harry: What is it?
Dumbledore: Love, Harry. Love.
Professor Dumbledore why Professor Quirrell disintegrated when Harry touch him.