But predicting the future is difficult and usually impossible. If things are bad, they'll tend to stay bad. If things are good, they'll tend to stay good. People often embroider these stand-pat predictions with splashy theories of how the world has permanently changed. Sooner or later, it stops mimicking the recent past.
People then realize that their assumptions are wrong, either too optimistic or too pessimistic.
Stock prices rise or fall. Typically, the process is gradual, because opinion changes gradually. Meanwhile, stock prices can remain too low or too high for years. Precisely the opposite. But just because much of the market's advance is sensible doesn't mean some of it isn't speculative.