Why did Snape want the Dark Arts job?

Question

There's a lot of talk about why Snape was denied the Defense Against the Dark Arts teaching job, but why did Snape want it in the first place?

He is shown to be very skilled at two branches of magic: Occlumency (enough to resist Voldemort himself) and Potions (as a student he figuratively and literally wrote the book on the subject) – and his attitude in the notation in the Potions textbook implies he took great pride and enjoyment in his Potion skills in particular.

True, he had an interest in the Dark Arts as a student, but surely he realized he would never be able to do "Dark Arts research" just by holding the teaching post (also, per JK Rowling Why did Snape love dark Magic so much?, Snape was interested in group membership more than the Dark Arts themselves). Furthermore, it was subtly implied that he lost his interest in the Dark Arts after he lost Lily because of them. Finally, Harry's favorite subject was Defense Against the Dark Arts, and as petty as it is, Snape maintained the sort of pride that would not enjoy pining for the teaching position that was Harry's favorite.

So why would he want the job at all?

Answer 1

'The Dark Arts,' said Snape, 'are many, varied, ever-changing and eternal. Fighting them is like fighting a many-headed monster, which, each time a neck is severed, sprouts a head even fiercer and cleverer than before. You are fighting that which is unfixed, mutating, indestructible.'
Harry stared at Snape. It was surely one thing to respect the Dark Arts as a dangerous enemy, another to speak of them, as Snape was doing ith a loving caress in his voice?
'Your defences,' said Snape, a little louder, 'must therefore be as flexible and inventive as the Arts you seek to undo. ...'
_{Harry Potter and the Half-Blood Prince, Chapter Nine - The Half-Blood Prince}

It seems pretty clear from that quote that Snape both loves the Dark Arts themselves - hardly news to anybody, I imagine, especially considering he (to quote Sirius, I think it was) came to Hogwarts knowing more Dark curses than most seventh years - but also the unique combination of skills required to combat them. He also seems to place a premium on branches of magic that require more than memorisation of incantations and wand movements (Charms, Transfiguration) or even seemingly very little actual magical ability (Divination, Care of Magical Creatures, History of Magic).

He expresses a similar view about his previous subject, Potions:

'You have no subtlety, Potter,' said Snape, his dark eyes glittering. 'You do not understand fine distinctions. It is one of the shortcomings that makes you such a lamentable potion-maker.'
_{Harry Potter and the Order of the Phoenix, Chapter Twenty-Four - Occlumency}

Since being successful at Defence Against the Dark Arts requires that you be "varied and ever-changing", I can easily see him viewing the subject as the pinnacle of magical ability (there's a reason that Aurors are considered "the elite" and very few Hogwarts students are accepted into the training), and thus gaining a huge amount of satisfaction from being the one in charge of teaching it.

Answer 2

I have two reasons to suggest for Severus being motivated to assume the role of DADA teacher.

The first and most important is to understand that Severus is indeed a very intelligent and gifted wizard placed in a very demanding role as double agent. He first went to Dumbledore in his grief over the death of Lily. Dumbledore would have helped him see the pain and suffering Voldemort was creating in his rise to power and offered Severus the opportunity to not only un-do some of that, but also to potentially avenge Lily's death. From that point on, his core purpose was working towards the diminution of Voldemort's power and influence so that he could be defeated.

The second is that, while Voldemort may have initially cursed the DADA position out of pique (if I can't have it, no one shall!), it also served a more long range purpose. By sabotaging the position, Voldemort reduced the chances of the upcoming generations of students producing wizards capable of challenging him. If he had managed to get the job, he could have recruited suitable candidates while also making sure the remainder would have been ineffectual in combat. Cursing the position at least gave him that second goal. If Severus became the DADA teacher, he would have been in a position to subvert that purpose while still keeping up the dual roles required of him.

So, in addition to being a spy and informant for Dumbeldore, Severus may have wanted to groom a generation of wizards to be more effective opponents of Voldemort.

Answer 3

I think what was said in that last comment; Snape wanted to prepare the next generation of witches and wizards to protect themselves from Voldemort and his Death Eaters, is at least part of the answer. I also believe that due to Snape’s own childhood full of bullying and abuse caused him to see the importance of learning how to defense oneself as a child. We can also see through his sacrifices, and the way he does things like jumps in front of Harry and the other children when Remus turns into a werewolf, that even if he dislikes Harry because of James he still believes protecting children is more important and worth risking his own life/well being. Even if he had been aware of the curse this may have motivated him to overrule self preservation in order to teach the children to defend themselves. Or perhaps he wanted to do as Lily did, motivated by his love for her, and was willing to sacrifice his own life if it meant protecting the children even if he did so indirectly by teaching them how to protect themselves. Or maybe the curse just didn’t matter to him because he didn’t have a happy life and lost the girl he loved and was still made to be a spy and double agent stuck between the man who killed his love and the man who promised to protect her and failed. Or on another strand of possibility Snape could have believed that Voldemort would have ended the curse if he managed to get one of his own Death Eaters into the position and was convinced they were teaching future generations of future Death Eaters the Dark Arts rather than defense.