Why does Luke feel bitter? (Chapter 7)

From the text, we can infer that Luke is bitter about his situation..... the father he has only met once, the role the gods play in his life, and possibly, not understanding the role that he would be required to play as Hermes' son.

“I don’t belong here,” I said. “I don’t even believe in gods.”
“Yeah,” he said. “That’s how we all started. Once you start believing in them? It doesn’t get any easier.”
The bitterness in his voice surprised me, because Luke seemed like a pretty easygoing guy. He looked like he could handle just about anything.