[Don't worry. He's asked that question to himself many times.]
There are worse things than gods. And right now those worse things want to take over. And, like it or not, the gods are our family. [Technically, so are the worse things...]
And maybe ... maybe they don't need me. Maybe they've never needed me. I've tried running away. But my path always crossed back, because I knew something, because I happened to be in the way... I don't know. Fate.
I'm not just fighting for the gods. The war right now, if we lose, the whole world will suffer. I'm not going to let a bunch of kids die just because they treated me badly. I'm not going to let the world fall because the gods can be selfish and cruel.