But, instead, many will worry. They worry the gains won't last and 2018 will deliver a loss.

Data says those worries are misplaced. After a good year, history tells us to expect another good year.

But, after a gain of 20% or more, the index gains an average of 11.2% in the next year. And the win rate jumps to 83.3%.

This means investors should be excited about 2018. They should invest aggressively until there is a clear sell signal. A close below the 200-day moving average could be a sell signal, although there are other, and better, signals.

Selling early means missing out on gains. And the gains after a good year could be substantial.

In one recent case, a winning streak in the stock market produced life-changing gains for investors.

Could that happen again? Yes, it can.

And that's important to remember. We could see big gains for a few years in a row.

So stay invested until the market turns down. None of us can afford to miss a bull market.