Dumbledore: Harry, do you know why Professor Quirrell couldn’t bear to have you touch him? 

[Harry shakes his head] 

Dumbledore: It was because of your mother. She sacrificed herself for you, and that kind of act leaves a mark. 

[Harry reaches up to touch his scar] 

Dumbledore: No, no. This kind of mark cannot be seen. It lives in your very skin. 

Harry: What is it? 

Dumbledore: Love, Harry. Love.