A steadily climbing market lulls investors into a false sense of security, convincing them to take more risk than they could otherwise stomach. Weeks like the one we just experienced are an important reminder of exactly how much risk you accepted when designing your portfolio. Volatility is scary. It's even scarier when you are in or near retirement, or you have under-saved, and you need the market to keep climbing to hit your number. On the other hand, investors who are still in the asset-accumulation phase are probably frustrated that the market bounced back before they could invest more capital.
Stocks may be the best-performing asset you can hold for the long term, but the price of that performance is volatility. If your head doesn't know that, or your stomach can't take it, then you'll likely miss the long-term benefits of ownership, because you'll sell at the worst time.