The Lightning Thief

From the text, we can infer that Luke is bitter because he is without purpose. He has been at the camp for a very long time.... five years.

“No prob.” Luke sat next to me, pushed his back against the wall. “Tough first day?”
“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.