From Yale Economics Professor Robert Shiller in the New York Times: Stock Market Mass Psychology
Canny stock investors are like judges in a quirky beauty contest. They aren’t looking for real beauty but for qualities that other people believe still other people will find beautiful.
Shiller invented the Cyclically Adjusted Price Earnings ratio, defined as the inflation adjusted stock price divided by the ten year trailing average of real earnings. He notes in this article that CAPE > 30 today, and has happened only two other times 1929 (80% loss from that period) and 1997-2002 (50% loss from that period).
He then observes that there were widespread newspaper articles noting the overvalued nature of previous bubbles, and that this hasn’t yet occurred for FANG stocks. Basically, the mass psychology of investors hasn’t turned against the bull market yet.