If this has already been answered, please forgive me:
What do you think Dumbledore's reason(s) is/are for not letting Severus take the Defense Against the Dark Arts job?
I don't really have a theory but these thoughts do pop into my head:
a) Because he's a spy for the Order, Dumbledore doesn't want to blow Snape's cover when he reveals all that he knows about the Dark Arts...or maybe...
b) If Snape practices the Dark Arts away from Voldemort, he (Voldemort) will somehow know and think that Snape will blow his cover...and kill him?
c) Snape knows far too much about the Dark Arts that would make anyone cringe. So, by keeping him as the Potions Master, he is safe from blowing either cover?
d) There is simply no other Potions Master to take over his job...thus, leaving Snape to always be the Potions Master.
Just some thoughts.... I'm still working it out lol.
Well I must be on my way, going on a small trip.
Later. And if you have anything to say, hit me.