When something bad happens to us, it's hard to face that fear again.
When I was six years old, I got stuck in an elevator. It only lasted about a minute, but for years afterward I always took the stairs.
Buddy, a golden retriever, had a terrifying experience when he ran into a glass sliding door thinking it was open. The poor guy never recovered, and now it takes a whole lot of convincing to get him to come in the house that way.