Bull markets climb a wall of worry, and this one has climbed a veritable Cliff of Insanity, with steep macro problems coming out of the Great Recession. So far the Bull has largely tracked increased earnings, but they are now lagging, particularly as the Dollar strengthens and profits on foreign earnings drop. In addition, many commentators have noted how the S&P correlates with the Fed Balance Sheet from QE (see chart), but that too has now ended.
What will drive stocks higher? Look no further than the Flight to Quality happening worldwide. Pundits have been surprised by the drop in US Treasury Yields after QE ended, but they shouldn't have been; the USD is rising sharply as flagging economies use Currency Wars to promote exports, driving other currencies down. One somewhat unexpected consequence is the scramble for better collateral; there is an estimated $9T USD carry trade worldwide, and as currencies drop in other countries, the need for collateral drives demand for US Treasuries and other US assets.
What may happen? The demand for USD should accelerate, and act like a liquidity pump like QE or the Greenspan Put before it in (for example) 1998-99, which drove the historic dot-com bubble. This pump will likely spill over into US equities and may lead to yet another bubble.