John Traynor, chief investment officer at People's United Advisors, said investors should watch two major events in 2018: midterm elections and simmering global trade tensions. "We think the earnings growth is very strong, we think the economy is very strong, but you're being buffeted by those two trends, and they will impact the second half of the year," Traynor said.

1. Midterm elections

As the November midterm elections grow closer, Traynor said, "You're going to see a lot of market volatility and turmoil."

Voting Poll Getty Images

2. Trade tensions

It's not just chatter, Traynor said global trade tensions could actually slow the economy. "The more the trade tensions roil the global markets and stay in the headlines, the more we're worried it will have a negative impact on business and consumer confidence," he said.

China's debt load is worrying investors and observers. STR | AFP | Getty Images