According to this article [1] money available for investment can be in stocks, cash, or bonds. The market goes up when people prefer stocks, down when they prefer cash and bonds. At present, the allocation to stocks is already higher than it has ever been, since 1950. Chart from FRED [2]. So if this theory is correct trouble is not far off.

[1] https://www.philosophicaleconomics.com/2013/12/the-single-gr...

[2] https://fred.stlouisfed.org/graph/?g=1Wc2g

My own theory is that at present inflation makes bonds unattractive. At some point a recession will kill inflation (and profits) and make bonds look better relative to stocks.