Waiting For The Other Shoe To Drop

I was just having this conversation with someone in our church. We've been doing this sermon series on all the times Jesus either did something to demonstrate that we don't need to fear, or He just said "Don't be afraid." The thing we were talking about is how odd it really is, that no matter how good God has proven Himself to be, and no matter how good things may be in our lives, we still have this nagging worry where we're waiting for the other shoe to drop. Why is that? Why do we still drag around that nagging feeling when the vast majority of our experience has been good, in fact more than good, and we've even seen God come to our rescue when the shoe has dropped in our past?I'm honestly asking you the reader: why do we have this nagging fear that the other shoe is about to drop?