First off, the S&P 500 Index is up about 4% as of May 18. Back at the start of the year, analysts tracked by Bloomberg forecast an 8.1% advance in the index. That's possible, but there are plenty of headwinds this year, including an almost certain hike in interest rates. It's also useful to consider that analysts have a historical bias toward bullishness.
Consider, too, that while the market has delivered an annual average of about 7.4% since 1990 that reflects the average of some big gains and big losses over the decades. Take a look at this chart below outlining the distribution of gains and losses in the Dow Jones Industrial Average.
Another worrisome trend: Corporate profitability is higher than its historical average at this point in the business cycle and it's starting to shrink. That's not good for share prices going forward if you think profits have peaked and there's little upside remaining. Morningstar analyzed the S&P 500's profitability—as measured by net income margin, operating margin, and return on equity (ROE)—and found that it is above its mid-cycle and long-term averages.