Because i think it is generally overpriced and has been propped up by the fed and record Margin borrowing / buying for a long time.
Look at the rounded top formed on the daily for S&P recently. Euro zone is in trouble and oils next leg down will drag the market down again. Too many red flags to be bullish on overall market. I just don't see much upside from here without a significant pullback.
All posts are my opinion based on my own DD. Always do your own research and never buy or sell based solely on anything I say.