I think it’s that people realize that the underlying issues that caused the financial crisis haven’t actually been fixed, and the Fed has been experimenting with policies that have never been seen before to prop up markets. While it’s worked so far it still feels like there are structural weaknesses that could cause massive problems, and just because things have been good doesn’t guarantee they won’t eventually fail, but timing is notoriously difficult.
"Policies that have never been seen before" seem to describe a good part of the last century, which has certainly had booms and busts, but the novelty has been more of a constant. We just don't have that long a history of modern economics. We've been flying by the seat of our pants the whole time.
