"Did you hear about the dementors too?" said Harry with difficulty. 

Lupin looked at him quickly. 

"Yes, I did. I don't think any of us have seen Professor Dumbledore that angry. They have been growing restless for some time -- furious at his refusal to let them inside the grounds.... I suppose they were the reason you fell?" 

"Yes," said Harry. He hesitated, and then the question he had to ask burst from him before he could stop himself." Why? Why do they affect me like that? Am I just --?"

 "It has nothing to do with weakness," said Professor Lupin sharply, as though he had read Harry's mind. "The dementors affect you worse than the others because there are horrors in your past and Amelia's that the others don't have." 

A ray of wintery sunlight fell across the classroom, illuminating Lupin's gray hairs and the lines on his young face. 

"Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them. Even Muggles feel their presence, though they can't see them. Get too near a dementor and every good feeling, every happy memory will be sucked out of you. If it can, the dementor will feed on you long enough to reduce you to something like itself... soul-less and evil. You'll be left with nothing but the worst experiences of your life. And the worst that happened to you, Harry, is enough to make anyone fall off their broom. You have nothing to feel ashamed of." 

"When they get near me --" Harry stared at Lupin's desk, his throat tight. "I can hear Voldemort murdering my mum."

 Lupin made a sudden motion with his arm as though to grip Harry's shoulder, but thought better of it. There was a moment's Silence, then -- 

"Why did they have to come to the match?" said Harry bitterly.