Desperate for Why

Stephen King gets a bit of a bad rap, but he's an incredible writer. There's a reason that he is the best-selling of all time, except for maybe Shakespeare and God. Right now, I'm reading Desperation. Check this quote:

"Why didn't you kill me like you did that guy back there? Billy? Or does it even make any sense to ask? Are you beyond why?"

"Oh s***, we're all beyond why, you know that."

My friend Mark tells me that economics is based on the idea that people act rationally given the information they have. I tend to agree. What do you think? Do we tend to make rational, if mis- or ill-informed, decisions? Or is it foolish even to try to explain why people act as they do?