Is the worst bear market since the Great Depression finally over? It sure looks that way. Defying critics, skeptics and conventional wisdom, the U.S. stock market has emerged from the agonizing, slow-motion crash of 2000-02 to advance 25% in the past 12 months -- putting it on track to record the first positive year of the new millennium. Popular companies such as Intel ( INTC) and Amazon.com ( AMZN) have done even better than that, respectively zooming 104% and 201% in the past year as investors have cheered their brilliant comebacks from the depths of despair. Overall, stocks of the new-school technology, biotech and retail companies that make up the Nasdaq 100 have more than doubled the return of the broad market -- scorching naysayers with a 55% gain since October 2002. If returns of this magnitude have taken you by surprise, welcome to the club. Thousands of households are still sitting on billions of dollars in cash that they withdrew from stock accounts during the two years of decline, swearing that they would never bet their hard-earned dollars on flimsy slips of paper again. And now they are wondering whether crooked pros on Wall Street have played them for fools once again, enticing them into the market with big gains just as prices are about to drop again. So is this unexpected gift about to vanish in a cloud of smoke, or is it really safe to own stocks again? To answer, you need to understand why the rally happened in the first place. Day in and day out, there are five different factors that govern the performance of stocks: the general mood of investors; business expectations for the near future; interpretations of the recent past; the supply and demand of stock and cash; and competition from other types of investments. And as fate would have it, all five have favored stocks in 2003.