Markets move in cycles and based on game theory. Everyone is risk averse and everyone jumps in when it appears "risk free". This is how prices would be bid up.
Stocks work like auction.
During Bull runs -> Highest payer - bids up the prices and the averages increase.
During Bears -> it's a fire sale. BUYER has an upper hand and takes the lower prices available.
It's human nature...
Game theory states you buy whilst you can else you will be left behind.
during "ATH" prices fly because prices are relative. Where the driver is the credit condition cycle (loose is good) and ofcourse ETFs.