Stock valuation is not rocket science. If you look at U.S. stock prices realistically, private debt is at an all-time high relative to GDP (on a total amount, per capita, and as a percentage of GDP)! The unemployment figure widely publicized is not high, but the U6 number which includes marginally attached workers is super-high (and has been for years). So things are not as good as many people think, especially regarding the future growth prospect of corporate earnings. Factor in the fact that the stock market has been propped up by a low interest rate policy, and we have a potential recipe for slow stock price growth going forward, if not a big downward move in U.S. stock prices. When interest rates do finally increase, watch out!
Another big issue is the algorithmic traders run the short-term stock market! Whenever there is a big short-term move in either direction, they are usually a big part of the cause, if not the sole reason. They are doing their best to get paid when the market goes up or down. When trading volume increases, regular folks often start to buy or sell following the lead of the algorithms, despite not necessarily understanding short-term price action driven by support or resistance levels, and written into many trading algorithms. These algorithms will change direction and push the short-term market a different direction in order to capture quick profits.
I believe people will significantly adjust their investment portfolios when interest rates rise bringing many overbought stocks down to their appropriate valuation.