This is a market feeding on itself. The chart above shows by how much US margin debt has accelerated away from norms, compared to the S&P 500 index in the past twenty years. The accelerating gap explains why the stock market began levitating away from earning growth from 2013 onwards (see second chart below on earnings). Note how the 2000 acceleration led to the immediate crash afterwards and how the 2003 acceleration was followed by the crash of 2007/8. That was before QE. Note then how in 2013, as QE began to be a worldwide programme, it accelerated again. The market now is entirely dependent on QE.
Ultimately, pumping cash into the economy by the Fed (note that gross national debt jumped $723 billion over just the past 12 weeks since Congress suspended the “debt ceiling” to $20.57 trillion, or 105% of GDP) has to go on at an accelerating rate to keep the market going. So the trap for QE is the exponential factor. If the market can’t be fuelled by debt exponentially it will reverse, and the higher it goes, the harder it will fall.
On the real side of the economy, Trump’s tax reduction plan (currently going through Congress)will not only backfire on the economy, but has scuttled the infrastructure plan (which is not going through Congress, nor will it), which was the idea that started the Trump stock market bull run in the first place.
The point about mania has always been that, from the perspective of all us mortals just standing by looking, it has the quality to last so long it draws everybody in.