Lessons From a Crazy Year in Financial Markets

Here is something many investors would have found difficult to believe during March’s stomach-churning selloff: 2020 would turn out to be a stellar year for the stock market.

The Dow Jones Industrial Average is near a record, slipping about 0.1% Thursday alongside the S&P 500 in muted holiday trading. The market for initial public offerings is flourishing. Just weeks ago, home-rental startup Airbnb Inc. made a stock-market debut so stunning that its chief executive was briefly left speechless on live television.

These are things that would be easy to imagine...