Fear of missing out may be causing most U.S. investors to overlook their best opportunity right now.Got the itch to dive headfirst back into the stock market? It would be understandable if you did. After all, the S&P 500 (^GSPC -0.33%) recently reached yet another record high, prompting fears of missing out on any further gains. Even something as simple as a new or bigger stake in the SPDR S&P 500 ETF Trust (SPY -0.33%) would be an easy way to get back in (or get in deeper).Before succumbing to temptation, ...