The uncomfortable reality is all Bull Markets, no matter how lengthy or robust, all expire.
As you have undoubtedly heard, the current Bull Market in U.S. stocks (S&P 500) is the longest in recent history--though some historians claim that Rome's SPQR Index rose steadily though most of Emperor Augustus' 40-year reign from 27 B.C. to 14 A.D., easily topping the current rally's 10-year run.
It's also possible that the longest Bull Market occurred circa 2580 B.C. in Egypt, as a result of quarry stocks soaring for decades during the construction of the Great Pyramid at Giza. The unprecedented outpouring of wealth for labor and materials boosted the stocks of a variety of sectors, from quarries to shipping to breweries slaking the thirst of the thousands of laborers toiling on the project for the better part of a generation.
Why do epic Bull markets that grind higher year after year finally expire?Analysts have various economic and financial reasons they tout: Treasury bond yields invert, debt begins to drag on growth, authorities raise interest rates too steeply, and so on.