The S&P 500 has seen an average annual return of 14% over the last few years, leading to overconfidence among investors. However, historical data shows a significant market decline from 2000 to 2009, where it fell by 57%. With a price-to-earnings ratio of 30, a reversion to the mean could result in a 50% decline. The low dividend yield also indicates potential risk, suggesting that the current bullish sentiment is unwarranted.