With the S&P 500 up for six straight weeks and counting, there is a genuine skittishness on the part of investors that the market has gotten ahead of itself and is due for a pullback. Two examples of the increased wariness regarding the market are the weekly Bespoke Market Poll, which has shown more bears than bulls for three straight weeks now, and the weekly bullish sentiment survey from the American Association of Individual Investors, which has declined for two straight weeks.
Only time will tell if investors will be proven right, as the S&P 500 is certainly overbought. That being the case, though, this year's rally to start the year is hardly unprecedented. Look back no further than last year at this time and you can see that the S&P 500 was actually up more YTD (7.39%) than it is this year (6.53%)! Last year, it wasn't until the S&P 500 was up more than 12% on the year before the index saw its first meaningful pullback.