Dumbledore looked very intensely at Harry for a moment, and then said, “I have a theory, no more than that … It is my belief that your scar hurts both when Lord Voldemort is near you, and when he is feeling a particularly strong surge of hatred.”
“But … why?”
“Because you and he are connected by the curse that failed,” said Dumbledore. “That is no ordinary scar.”
- Goblet of Fire.