Dumbledore Explains the Defense Against the Dark Arts Curse
Page 446
Back in the present, Dumbledore explains to Harry that ever since Lord Voldemort was refused the Defense Against the Dark Arts position, Hogwarts has been unable to keep a Defense Against the Dark Arts teacher for more than one year. This curse seems to be Lord Voldemort's retaliation for Dumbledore's refusal, further solidifying his hatred for Hogwarts and its headmaster.
