“I feel I owe you another explanation, Harry,” said Dumbledore hesitantly. “You may, perhaps, have wondered why I never chose you as a prefect? I must confess… that I rather thought… you had enough responsibility to be going on with.”

Harry looked up at him and saw a tear trickling down Dumbledore’s face into his long silver beard.

-page 844, Harry Potter and the Order of the Pheonix