Public companies that are listed on the stock market are traded Monday through Friday. Over the very long term, publically traded stocks have a distinct tendency to go up, of which, some of that has to do with inflation, some of that has to do with the underlying growth, value, and earnings of a given stock, some of that has to do with interest rates as well as other monetary policies, and some of that just comes down to speculation. Although, there are pundits that claim that the market is "efficient," in the sense that since there are buyers as well as sellers for every given stock, and the pertinent information in regards to the economy and a given company, is public knowledge, than their belief is that the subsequent price of the stock, clearly reflects such. Yet, if this was really true, stocks wouldn't rise and fall as much as they do, especially when on most days, there really isn't much actionable information going on in regards to a specific stock. Further to the point, anyone or any institution that opens up a brokerage account, are not controlled in regards to their decision making process as to whether a stock is "fairly priced" or not, and hence many stocks can be priced at a price range that doesn't match all that well with the fundamentals of that stock.
A case in point, is that the USA stock market as of 2020, is in most of its indexes at an all-time high, or has hit that high this year, which given the fact that the GDP of the United States has been in a low-growth phase for over a decade, doesn't make a lot of sense, to begin with. Further to the point, the United States, will have a significant negative growth rate for 2020, so that, when the economy is shrinking, and because the unemployment has risen tremendously, it would seem, even with very low interest rates, and other mitigating factors, that the stock market should reflect that profit margins along with growth rates are being impacted in aggregate, in a negative manner.
The error that many investors make in regards to the purchase of common stock is to not readily understand very well what they have purchased. While it is true that as a common shareholder, they have a minute ownership percentage in a given company, the value of those shares, might not have a lot to do with the true intrinsic worth of a given company, and instead have a lot more to do, with projections of growth and value that are often very problematic. Again, when a given country's yearly GDP is 2.5% or thereabouts, but the stock market is consistently returning around 8-10%, yearly, then that would seem to indicate that the pricing of those stocks being traded in aggregate, is not consistent to what is really happening in the economy in reality.
So then, as might be expected, when stocks appear to be overvalued per most metrics, then the expected future return of those stocks, are going to be minimal, because those stock prices have gotten ahead of their reasonable underlying worth, via speculation, that has been too overly optimistic or too greedy. History, is replete with speculation upon speculation, that have done exceedingly well, only to at some later point, collapse. After all, when there are too many sellers, and there aren't any willing buyers, prices go down until some sort of equilibrium is reached.