Faith
She was having a crisis of faith... so to speak.
Maybe at one point during her life, slaying had been the be all and end all. But something had changed... she had changed, and now it was too hard to change back. Too many things had happened and now, it wasn't easy to forget them and move on, as she had always been able to do.
Somewhere along the line, Faith had lost the will, and the inclination to fight.
Maybe it was because she didn't want to end up like Angel.
Angel died for the greater good, fighting the good fight, and no one would ever know. No one would know how many times he'd saved the world, not including the times he tried to end it. But he had died for a cause that he felt was worth fighting for.
But was it really?
Now that he was gone, it seemed like such a waste. Humanity's goodness and it's protection was a lost cause. Trying to preserve the innocence and safety... wasn't that just a losing battle anyway?
Sure, she fought evil, but evil kept coming back.
And it got stronger each time.
If the Powers That Be, the Elders and the Watcher's Council were all depending on one girl to save the world... they were asking for too much.
Faith had closed one Hellmouth, and she'd nearly died to do it.
She'd technically died a few weeks ago, but not really, because she was alive and kicking in Valhalla. On Earth, she'd died, but in the different dimension, nothing had changed. And so when she, from the different dimension, had come back... she was still... that.
She didn't want to die fighting for a cause that she no longer believed in. And now, she wasn't even sure if she'd actually ever really believed in it. Sure, she foiught for it, on the side of it, but had she ever really cared?
And on that thought, was it wrong that she didn't care? Did that make her an awful person because she didn't care about the fate of humanity?
Maybe they'd chosen wrong.
Sorry, wrong number, try again...
Maybe Faith was the wrong type of girl for the Slayer gig. She was the dark slayer, the evil one, the one who looked out for herself and herself only. The reluctant hero.
This cause was a cause that she didn't care about.
And that was her problem.