This excerpt[1] from "Surely you're joking, Mr. Feynman" comes to mind. Detailing a stage where Feynman lost his passion for physics and regained it by studying things purely for the satisfaction of his curiousity and the enjoyment of the process.

  He says, "Feynman, that's pretty interesting, but what's the importance of it?"
  "Why are you doing it?"

  "Hah!" I say.
  "There's no importance whatsoever. I'm just doing it for the fun of it."

[1]: http://www.physics.ohio-state.edu/~kilcup/262/feynman.html