It's always been regret. That's why the end game is the Fortress of Regrets, and the final dialog stuff is all about regrets and how it changed him.

There's a point when you're asked the question and you can answer, and belief has a strong evidence backing it based on the shit you talk to the final boss about how belief can move planes.

But thematically it's regret. The question was not designed for all of mankind. It wasn't a rhetorical question, but one with real consequences specifically about how to go about actually changing the fundamental nature of one man in particular, and in the context of the story that one man had his answer.