Ooh! I know this one! Nowhere, canonically, does Snape confirm that he truly wants the job, but JKR did so in interviews. She said, "Snape asked for the position when he came to work for Dumbledore, but Dumbledore was concerned about him working with Dark Magic, and so said, 'Let's see how you do with Potions for now.'" (paraphrased) I actually watched this interview; it was the one with Stephen Fry in front of that huge audience right after OotP was released.