Albus 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. (Harry touches his scar) No, this kind of mark cannot be seen. It lives in your very skin.

Harry Potter: What is it?

Albus Dumbledore: Love, Harry. Love.

From Harry Potter and the Sorcerer’s Stone (2001)