CNBC, day trading, the Motley Fool, Silicon Investor -- not since the 1920s has there been such an intense fascination with the U.S. stock market. For an increasing number of Americans, logging on to Yahoo! Finance is a habit more precious than that morning cup of joe (as thousands of SBUX and YHOO shareholders know too well). In Irrational Exuberance, economics professor Robert J. Shiller examines this public fascination with stocks and sees a combination of factors that have driven stocks higher, including the rise of the Internet, 401(k) plans, increased coverage by the popular media of financial news, overly optimistic cheerleading by analysts and other pundits, the decline of inflation, and the rise of the mutual fund industry.