“The VIX is too low, valuations are too high, the recovery is too old and the Fed is tightening,” said Jim Paulsen, a veteran market strategist for the Leuthold Group in Minnesota. “For an old market dude like me, that is a scary list.”

For several years now, Mr. Paulsen has been pleading with his clients to embrace the bull market. He has touted the breadth of the surge, in which no single sector has dramatically outrun another on the way up. And he has argued that worldwide growth with little inflation represents an unusual buying opportunity.

When he meets with clients or presents his views at conferences, though, the vast majority of the questions he receives are about what will ultimately bring the market crashing down.

“No one ever asks me when the S.&P. is going to blow past 3,000,” he said of the benchmark stock index which ended trading on Friday at 2,587.84.

In fact, many analysts say that this so-called wall of worry is a positive sign. Investors may be piling into stocks and bonds, the thinking goes, but they are doing it with a measure of hesitation, which prevents some of the excesses that preceded previous market corrections.

That is not to say the market is without froth.

Since early 2009, the market capitalizations of Amazon and Apple, have soared from $26 billion and $74 billion to $532 billion and $872 billion.

The surge in the price of Bitcoin strikes many observers, including the billionaire investor Warren Buffett, as a classic portent of a bubble ready to pop.