The U.S. stock market is sitting within a fraction of a percent of a new all-time high. You might expect that to feel reassuring, but for many investors, it has the opposite effect: A nagging sense that things have gone a little too well and the other shoe is about to drop. That uneasy feeling is worth examining. Because how you respond to it will likely matter far more to your retirement than whatever the market does next. In today's email, I'm sharing one of the best investing analogies and...