I love that those 2 explanations directly follow each other:
rayvt:
Ken Fisher says that the market takes into account all known information.
bankersfate:
Greed.
FOMO.

No conflict. One of the pieces of information is that some people are feeling greedy : )

I think a very big factor in recent years is that a much larger, and now probably dominant, fraction of equity buyers/holders are completely price insensitive. If you are an equity fund manager (managed or indexing), or have a bounded minimum equity exposure mandate, if money rolls in the door you have to be a buyer, even if you know it's a bad idea. A lot of money arrives every month through pension contributions and the like. There just aren't that many price-aware sellers left, so from whom will they buy?

This doesn't say anything specifically about the current bull market nor the current rally within it, it's just an upwards pressure that continues to exert a tail wind.

This effect can go on for an extremely long time, giving itself positive feedback, but it also might eventually fall into the category of "if something can't go on forever, it will stop". The whole machine could one day go into reverse. Though we might not live long enough to see it.

Jim