You'd think by now I would have learned not to believe anything Dumbledore says uncritically. Let's take another look at the passage where he confirms the curse's existence:
You'll notice that he never actually says Voldemort cursed the position. He doesn't even say he suspects Voldemort cursed the position. He just says that no one has lasted longer than a year in the job since and lets us draw our own conclusions. Even for a man who routinely uses academic hedging and waffling, that's... curious. He isn't afraid to "believe" or "suspect" things like the exact means by which Merope Gaunt got Tom Sr. to run off with her, so why so circumspect here?
Maybe because the means by which Voldemort proved he wanted the job wasn't as straightforward as cursing the job himself.
I had previously speculated that the DADA curse worked by taking an existing "don't seriously injure the students or put them in mortal peril" protection in the castle and cranking it up to eleven. That does seem to be the mechanism of the curse, but the hypothesis has some gaps if we assume Voldemort did it. How did he gain control over part of the castle's inherent magic? Why didn't Dumbledore figure out how the curse worked earlier, or if he did, how to dial it back down to normal levels? Why didn't he abolish the position and replace it with a new one covering the same subject matter to see if that worked? Why does he risk his own agents, ones he presumably doesn't want killed (yet), multiple times? Etc.
Well. Those problems would disappear if Dumbledore cursed the position.
Picture him sitting in his office after Voldemort left the job interview. He believes Voldemort will try again, one way or another; this must have been a reconnaissance mission to see how Dumbledore is doing these days. Suppose he manages to get Dumbledore ousted as headmaster by controlling the Board of Governors (as Lucius did temporarily years later for his own purposes), and his replacement is more, ah, persuadable when it comes to hiring decisions? Or suppose he sends an agent in to kill Dumbledore? Dumbledore might be confident that he would defeat the agent, but there might be enough death and destruction beforehand to bring him back to Possibility #1, getting fired. Or maybe the Board of Governors can override the Headmaster's hiring decisions in certain circumstances. We know the Ministry can appoint someone if they think they need to; suppose Voldemort gets control of the right person at the Ministry to get himself or his agent appointed?
No, he can't let that happen. There must be some way to limit the damage--or at least the time period in which Voldemort/his agent could inflict such damage.
And that's when he remembers either an obscure curse or an obscure bit of the castle's magic which is currently inactive or operating at a very low level of sensitivity, and decides to make sure that no one can hold the job for more than a year. Even if Voldemort succeeds in getting the job somehow, his victory will be short-lived.
The next two or three years, Voldemort does send agents to get the job, and each is disposed of with varying degrees of damage. This is what proves Voldemort really wanted the job.
Voldemort soon realizes what's up, more or less, and decides to focus on his other projects, like recruiting more Death Eaters, making Inferi, and playing with ancient magic. He's got time, after all, and he decides that as inconvenient as it will be to have to train young recruits to fight properly, at least it means his enemies won't be able to fight back very well. That's not a bad Plan B. Maybe every few years, he tries again to see if a new agent can circumvent the curse and cause a little trouble for Dumbledore.
But most of the DADA teachers are ordinary people who don't do much harm, and so the curse disposes of them in fairly mild ways. Mild enough that people only occasionally suspect there is a curse, and generally chalk the turnover up to the teaching schedule being brutal. Not everyone can handle the workload as well as veteran teachers like Slughorn, right?
So when Quirrell and then Lockhart suffered much worse curse-induced rebound than usual, Dumbledore wasn't worried about hiring Lupin and then Moody, because he knew that the curse wasn't just ramping its intensity in general and whatever happened to them would be minor. (At least, it would if Lupin really weren't helping Notorious Mass-Murderer Sirius Black return Voldemort to power; if he wasn't, Lupin would come out of the year with a good cover story for becoming a werewolf spy, and if he was, Dumbledore would find out when the curse killed him or something equally terrible.) In Year 6, of course, he knew from the start the most likely reason Snape would have to leave the job, and it was all right with him.
Of course, this has the unfortunate side-effect that students learn magical self-defense very poorly for nearly forty years, but Dumbledore can tell himself that they'll pick things up in other classes, and it's better than whatever Voldemort would have planned for them. It's for--say it with me--the greater good!
There. Does that work better?
"Oh, he definitely wanted the Defense Against the Dark Arts job," said Dumbledore. "The aftermath of our little meeting proved that. You see, we have never been able to keep a Defense Against the Dark Arts teacher for longer than a year since I refused the post to Lord Voldemort."
--HBP Chapter 20, "Lord Voldemort's Request
You'll notice that he never actually says Voldemort cursed the position. He doesn't even say he suspects Voldemort cursed the position. He just says that no one has lasted longer than a year in the job since and lets us draw our own conclusions. Even for a man who routinely uses academic hedging and waffling, that's... curious. He isn't afraid to "believe" or "suspect" things like the exact means by which Merope Gaunt got Tom Sr. to run off with her, so why so circumspect here?
Maybe because the means by which Voldemort proved he wanted the job wasn't as straightforward as cursing the job himself.
I had previously speculated that the DADA curse worked by taking an existing "don't seriously injure the students or put them in mortal peril" protection in the castle and cranking it up to eleven. That does seem to be the mechanism of the curse, but the hypothesis has some gaps if we assume Voldemort did it. How did he gain control over part of the castle's inherent magic? Why didn't Dumbledore figure out how the curse worked earlier, or if he did, how to dial it back down to normal levels? Why didn't he abolish the position and replace it with a new one covering the same subject matter to see if that worked? Why does he risk his own agents, ones he presumably doesn't want killed (yet), multiple times? Etc.
Well. Those problems would disappear if Dumbledore cursed the position.
Picture him sitting in his office after Voldemort left the job interview. He believes Voldemort will try again, one way or another; this must have been a reconnaissance mission to see how Dumbledore is doing these days. Suppose he manages to get Dumbledore ousted as headmaster by controlling the Board of Governors (as Lucius did temporarily years later for his own purposes), and his replacement is more, ah, persuadable when it comes to hiring decisions? Or suppose he sends an agent in to kill Dumbledore? Dumbledore might be confident that he would defeat the agent, but there might be enough death and destruction beforehand to bring him back to Possibility #1, getting fired. Or maybe the Board of Governors can override the Headmaster's hiring decisions in certain circumstances. We know the Ministry can appoint someone if they think they need to; suppose Voldemort gets control of the right person at the Ministry to get himself or his agent appointed?
No, he can't let that happen. There must be some way to limit the damage--or at least the time period in which Voldemort/his agent could inflict such damage.
And that's when he remembers either an obscure curse or an obscure bit of the castle's magic which is currently inactive or operating at a very low level of sensitivity, and decides to make sure that no one can hold the job for more than a year. Even if Voldemort succeeds in getting the job somehow, his victory will be short-lived.
The next two or three years, Voldemort does send agents to get the job, and each is disposed of with varying degrees of damage. This is what proves Voldemort really wanted the job.
Voldemort soon realizes what's up, more or less, and decides to focus on his other projects, like recruiting more Death Eaters, making Inferi, and playing with ancient magic. He's got time, after all, and he decides that as inconvenient as it will be to have to train young recruits to fight properly, at least it means his enemies won't be able to fight back very well. That's not a bad Plan B. Maybe every few years, he tries again to see if a new agent can circumvent the curse and cause a little trouble for Dumbledore.
But most of the DADA teachers are ordinary people who don't do much harm, and so the curse disposes of them in fairly mild ways. Mild enough that people only occasionally suspect there is a curse, and generally chalk the turnover up to the teaching schedule being brutal. Not everyone can handle the workload as well as veteran teachers like Slughorn, right?
So when Quirrell and then Lockhart suffered much worse curse-induced rebound than usual, Dumbledore wasn't worried about hiring Lupin and then Moody, because he knew that the curse wasn't just ramping its intensity in general and whatever happened to them would be minor. (At least, it would if Lupin really weren't helping Notorious Mass-Murderer Sirius Black return Voldemort to power; if he wasn't, Lupin would come out of the year with a good cover story for becoming a werewolf spy, and if he was, Dumbledore would find out when the curse killed him or something equally terrible.) In Year 6, of course, he knew from the start the most likely reason Snape would have to leave the job, and it was all right with him.
Of course, this has the unfortunate side-effect that students learn magical self-defense very poorly for nearly forty years, but Dumbledore can tell himself that they'll pick things up in other classes, and it's better than whatever Voldemort would have planned for them. It's for--say it with me--the greater good!
There. Does that work better?