"When you are born," the golem said softly, "your courage is new and clean. You are brave enough for anything: crawling off of staircases, saying your first words without fearing that someone will think you are foolish, putting strange things in your mouth. But as you get older, your courage attracts gunk and crusty things and dirt and fear and knowing how bad things can get and what pain feels like. By the time you're half-grown, your courage barely moves at all, it's so grunged up with living. So every once in a while, you have to scrub it up and get the works going or else you'll never be brave again.
The Girl Who Circumnavigated Fairyland in a Ship of her Own Making