Big buyers are not coming in at this level. The big boys know this rally is extended. Those remaining aren't selling due to their euphoric sentiment. Retailers and latecomers buying dips. If they can't bring this market higher, the bears will take over at the first sign of blood. Kind of at the edge right now but hasn't been confirmed either way.
As we go into the summer the volume will dry up even more. Less and less buyers and well that leaves more and more sellers. Once certain levels of the S&P support are broken the selling will accelerate (875, 825, and then the 50dma). JMHO!