The U.S. job market continues to blow through expectations, generating 200,000 new jobs month after month and driving unemployment far below what economists thought a decade ago was the lowest possible level.

The main reason is that the economy tends to keep creating jobs until interrupted by a recession. The current expansion has now lasted a record 10-plus years. So long as the usual recession triggers—rising inflation and interest rates, or financial excess—remain absent, job creation should continue.

Yet...