Well, my current theory is that he's tired of fighting, misses Severus, wants control back over his own life, is fed up with the idiocy of everyone around him, cannot believe that a Griffindor is in charge of him(and Hogwarts), his back is killing him, oh, and to top it all off, he has to teach Weasleys, mudbloods, and the Boy Who is a Prat.
Damn, you know, that makes sense. I'd be snarky. Okay, I'd be crying in a corner, but you know...
I have problems with that too sometimes. I can see so clearly where I want to be, but no clue how to get there. I generally just start writing and it gets there eventually. Or it gets somewhere else, which sometimes is even better.