Don’t worry about stock-market volatility: It is perfectly normal. Do worry about how stocks got so high to start with because it is evidence of an economy still abnormally dependent on low interest rates and richly priced assets.

The 2,271-point drop in the Dow Jones Industrial Average in the week through Monday, a decline of 9%, didn’t even meet the usual 10% threshold for a correction. Tuesday’s 567-point rebound didn’t make the top-500 daily increases by percentage. Wednesday’s 509-point swing between high and low was...