S&P 500

1. Stocks are historically expensive
The stock market could crash because equity valuations are historically very pricey. As of Dec. 29, the Shiller price-to-earnings ratio for the S&P 500 -- a P/E ratio based on average inflation-adjusted earnings from the previous 10 years -- hit nearly 34. That's more than double its mean and median over the past 150 years, and it's the second-highest reading next to the dot-com boom in the late 1990s and early 2000s. Historically, when the Shiller P/E ratio gets above 30, bad things happen (i.e., the market crashes).

2. Emotions get the better of investors
Never underestimate the power of short-term traders overreacting to a news event. Over the long run, operating earnings growth is what drives the stock market higher. But in the short run, investor emotions tend to whipsaw day-to-day market activity. As we saw in March, it doesn't take much to completely upend investor confidence and send emotional investors into a panic. It's possible this happens again in 2021.