"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."
-- Albus Dumbledore (HBP20)
When Professor Merrythought of Hogwarts retired after 50 years as professor of Defence Against the Dark Arts, Tom Riddle as Lord Voldemort applied to Dumbledore for the teaching position, but was turned down flat (HBP20). After that, no teacher who took the job lasted more than one year, which created a staffing crisis and a string of often-mediocre professors.
The rest of the staff realized the job was jinxed, as Hagrid told the children in Harry’s second when Gilderoy Lockhart was the “only one” willing to become DADA professor (and not a very good one at that) (CS7).
Rowling has stated that after Voldemort was killed, the jinx was broken and Hogwarts could keep a DADA teacher longer:
Meredith Vieira: Do you-- do Ron and Hermione or Harry ever return to Hogwarts in any capacity?
JKR: Well, I can well imagine Harry returning to give the odd talk on-- on Defense Against the Dark Arts. And-- I-- and, of course, the jinx is broken now because Voldemort's gone. Now they can keep a good Defense Against the Dark Arts teacher from here on in. So that aspect of the-of the wizarding education is now provided for (Today1).
The jinx seemed to bring out the worst in the professor who held the job, to exaggerate character flaws that would lead to some type of downfall. So the reasons for leaving were usually quite serious:
- Quirrell: Weak and unable to fight, Voldemort possessed him and forced him to attack Harry, after which he died
- Lockhart: Constant lying and stealing memories from others led to him obliviating himself
- Lupin: Lycanthropy and being loyal to Sirius made him forget to drink his Wolfsbane Potion
- Moody: A fake eyeball, numerous scars and a paranoid personality enabled Barty Crouch Jr. to impersonate him
- Umbridge: Her hatred of children and centaurs came on full force as Hogwarts High Inquisitor
- Snape: His love for Harry's mother Lily led him to become a double spy. Forced to kill Dumbledore (by request) he then fled to Voldemort so he could be made Headmaster of Hogwarts the following year.
- The Carrows: Evil Death Eaters, they forced Pureblood students to torture other children until captured at the Battle of Hogwarts
While Harry believed that Snape craved the DADA job due to a comment by Percy Weasley at the first Sorting Feast in First Year (PS7), Snape had to know the job was jinxed. In second year, Hagrid said that Lockhart was "the on'y man for the job ...an' I mean the on'y one." However, to keep his cover with Voldemort and the Death Eaters, Snape dutifully applied for the job every year, as pointed out by Umbridge during his teacher evaluation in fifth year (OP17). This gave him cover later when defending his position at Hogwarts as "Dumbledore's pet," as Bellatrix Lestrange called him at Spinner's End:
"He wouldn't give me the Defense Against the Dark Arts job, you know. Seemed to think it might, ah, bring about a relapse ... tempt me into my old ways." (HBP2)
That was a plausible excuse without having to bring up the jinx on the job. Indeed, Snape never took the job until the last possible year, as Dumbledore was dying from his Horcrux-cursed hand (DH33). All of this background information on the jinxed DADA job undercuts the idea that Snape was always jealous because Lupin had the job instead of himself or that he "got Lupin sacked" out of spite, because he always knew Lupin would only last one year anyway.