NEW YORK - Somebody on a bus asks a friend, "How about that stock market?" The response: "Unbelievable." Caribbean vacationers lounging poolside check their Blackberries for stock prices. Suburban gym members chat about the latest market gains during their morning workouts.

Welcome to the 2009 bull market - or so many people think. They're buying up shares of everything from Google Inc. to Bank of America Corp. at a pace not seen since the 1930s. Since March, the Dow Jones industrial average has jumped 57 percent and the Standard & Poor's 500 index has gained 62 percent.

Investors are betting on a strong economic recovery. But here's the problem: Good news ahead could be bad news for the bull.

To understand why, consider the very thing that has boosted the market. The U.S. government has spent nearly $1 trillion to stimulate the economy and the Federal Reserve has maintained a policy of keeping interest rates near zero.

Those will disappear as the economy's health improves, potentially halting the bull market by taking away what has been its crutch - sources of cheap and plentiful money.

"Pretty soon the easy money phase could be behind us," said Hugh Johnson, chairman and chief investment officer of Johnson Illington Advisors, an investment firm in Albany, N.Y.

The government has plunged big money into the marketplace, through tax cuts, construction projects and other measures. At the same time, low interest rates have invigorated stocks by reducing borrowing costs and bolstering corporate profits.

The low rates have also knocked down the returns of other short-term investments, like government bonds and money-market funds. Since people aren't getting high returns on those investments, they're buying stocks.

Stocks are risky because they don't guarantee a return, and the recent bear market shows how deeply share prices can drop. From October 2007 through March, the Dow industrials lost 53 percent.

"The Fed is forcing everyone to take risk by buying stocks because if you don't take risk, you will be earning nothing on your money," said Ed Yardeni, president and chief investment strategist at Yardeni Research.

Yardeni said his clients, which include pension funds and institutional investors, feel like they don't have a choice but to buy stocks right now. He sees lots of "fully invested bears" - investors who don't believe that investing in stocks makes sense right now because of the state of the economy, but they are buying anyway because they worry they might miss out on a bull run.

The Dow is trading above 10,000 for the first time since October 2008, though it is still 27 percent below its peak two years ago.