First, I give you a chart created by Gary over at the Smart Money tracker.
I won't repost/steal everything Gary said. Go to his blog by clicking on this link and read.
The short story, the market is WAAAY out of wack above the 200 DMA. Revert to the mean is the worry by any prudent investor.
The second image is how the S&P 500 from 1973 to today. Just to put into context where the historical highs and our current position, and some general trend lines.
The last image is most critical. I really doubt Ben Bernanke over at the Federal Reserve bank wants to show a change in the 30 year bond rates, from declining to increasing. A stock market crack would drive rates down as it chases people into bonds. Chart from "TheChartStore.com". Another great pay service.