Stocks are on track to buck the historical declines we typically see in September, with the major indices expected to post their best September since 2013.

"I think the old adage is the market likes to frustrate the most people," said Doug Sandler, global strategist at RiverFront Investment Group. "Most people are waiting for a pullback - as a result we sort of get this frustration where as it goes higher people feel like they're going to get left out."

The S&P 500 is up some 12% since the start of the year. 

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