Very simple: stock price at end of day is demand/supply

When you have more buyers than sellers, stock price moves up. If market keeps going up without any meaningful pullback, more and more buyers are scuked into the market, more and more people will use margin to leverage their positions. 

Eventually, you run out of buyers, and market crashes hard

This is decided by human nature, buried in our DNA, and can never be changed

This is also a reason why elliot wave is so effective in judging the mid to long term direction of the market.