Investors are bullish, but there's still cause to be 'nervous,' trader says

Investors are bullish on the market, but there are still reasons to be concerned, one trader told CNBC on Monday.

"In general, if you look at the sentiment out there, it's pretty bullish," Ian Winer, director of equity trading at Wedbush Securities, said on "Closing Bell." "All of a sudden it just feels to me like everybody's playing the same game, you've got the same five names taking on a bigger and bigger percentage of the market every single day, and I'm a little nervous."

The U.S. stock market closed higher as President Donald Trump continued traveling in the Middle East, stopping in Israel on Monday.

When asked if Trump's closer ties to Saudi Arabia in the form of a nearly $110 billion arms deal was bullish for the market, one trader said that it was certainly a factor.

"Earnings expectations are going up," said John Manley, chief equity strategist at Wells Fargo Funds Management. "Mr. Trump arrived at the same time that better earnings did so I'm not sure that he's to be credited for that."

"Whenever President Trump says something and it comes across as being pro-business, that can't hurt," Manley added.

Defense stocks rose on Monday, with the iShares U.S. Aerospace & Defense ETF (ITA) closing nearly 1 percent higher.

However, Winer said he was a little nervous, noting that a few alarming headlines, like those about FBI Director James Comey's firing, could change sentiment.

"This momentum could reverse course real quickly, and you could see a nice dip in the S&P," he said.

The S&P 500 rose 0.5 percent, with information technology leading advancers, while the Nasdaq gained 0.82 percent.

The Dow Jones industrial average advanced about 90 points, with Boeing and 3M contributing the most gains.