The Standard & Poor's 500 Index already has risen 22.5 percent this year, and more gains might follow, but "stocks are not cheap," Michael Farr, president of investment management firm Farr, Miller & Washington, writes in a commentary for CNBC.

"If the rule is to buy low and sell high, this is not low," he says.

In December 1996, then Federal Reserve Chairman Alan Greenspan questioned whether stock investors were suffering from "irrational exuberance."

Editor’s Note: 5 Reasons Stocks Will Collapse . . .

According to Farr, "he was right. Stocks, by historical measures, were expensive." Of course, they didn't fall until 2000.

"The lesson is that expensive stuff can become a lot more expensive," Farr explains.

"Stocks are expensive and are becoming more so every day. This can last a long while."

Some pessimists say stocks are in a bubble. But, "market bubbles rarely occur when the consensus is worried about bubbles," Farr writes. "It's usually after the naysayers are dismissed forever as being old-fogy, pessimistic doom-and-gloomers that the bubbles eventually pop."

Farr isn't the only one worried about stocks.

"A lot of people are nervous by how strong the market has been this year," Patrick Kaser, a portfolio manager at Brandywine Global Investment Management, tells Bloomberg.

"There is still skepticism about how the economy is really doing and whether these gains are from artificial factors, like the Fed, or from real strength in company results."

Editor’s Note: 5 Reasons Stocks Will Collapse . . .

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