Your fears are as vague and uncertain as anything else. What you're afraid of can't be cured by staying away from a stepladder or stepping on a bug. They're unknowns in the dark.

[Insanity. Losing control. And Rei supposes losing the people you love is a specific fear, a very real one, but she won't name them out loud when Castiel has difficulty enough coping with emotion, let alone what he fears.]

That's a lot scarier than something you can see, feel, hold. Because you don't know when it'll come.