It always does? It's partly a psychological phenomena and partly just a question of running out of greater fools. The first one is that once there's a change in mood for some reason (some event happens, e.g. a recession which in some ways is its own psychological phenomena) everyone convinces themselves that equities are too expensive, the prices start declining and that feeds a further decline. Just look at the GME stock as a microcosm of that situation.

In terms of the greater fool thing, you can only keep buying and selling the same thing for a 10% profit every year without eventually running out of people who will buy it. I guess see also the GME stock as a microcosm of that?

And so, all bubbles must burst.

I don't think it needs to burst. Though there will certainly be times when it goes down there is no reason it'll even go below current prices.

Historically prices have always gone up, so as long as you aren't near a peak it's probably the lowest the total market will ever be.