I've noticed a similar thing about myself when I'm making plans with friends. They'll say, "Are you coming to XYZ event tomorrow?" I've noticed that I very rarely answer with "Yes" or a common equivalent even when I fully intend to go. In the back of my mind I'm thinking, "How could I possibly know whether I will be there tomorrow." I could die, be involved in an accident, or my transportation could be severely delayed. I usually respond with something like "Probably" or "I'll definitely try to make it". It doesn't have the dramatic impact that this story did, but it echoes the same way of thinking.