The recent ten-year bull run in the US stock market has reached a dangerous height.

First, let's look at the numbers—by September of this year, the ten-year rolling return of the S&P 500 has surged to 250.9%, matching the golden periods of the 1950s and from 1977 to 1987. What does this mean? Since 1930, it ranks fourth.

But don’t rush to cheer. There were two crazier times in history: the internet bubble from 1990 to 2000, which rose by 390.4%; and going further back, the "Roaring Twenties" from 1919 to 1928, which rose by 381.3%. How did those two end? You know.

The market is now divided into two camps. The bulls say: "It has only risen two and a half times, far from four times. The increases in the 1950s and 1980s were similar; didn’t they continue to rise afterwards?" The bears focus on valuation warnings: "Don’t just look at the price increase; look at the price-to-earnings ratio and the Shiller cyclically adjusted price-to-earnings ratio, which are already approaching the bubble levels of 1929 and 2000. Remember Black Monday in 1987? It also collapsed suddenly after a similar increase."

There is a detail worth noting from a technical perspective: if this year ends with a double-digit increase, it will be the sixth time in seven years—such a consecutive bull run has only occurred nine times in history.

In plain terms, this bull run has not yet reached the level of madness seen in the 1920s or 1990s, but valuations are already ringing alarm bells. Bulls believe the momentum is still there, while bears think a reversal could happen at any moment.

For ordinary investors, what they should focus on now is not betting on the top or chasing the rise, but closely monitoring two indicators: whether the valuation multiples continue to expand, and whether market sentiment has shifted from greed to hesitation. History tells us that the frenzy at the end of a bull run often comes suddenly and ends even more abruptly.