Just about every investor in the stock market is there as an investor primarily to make money on their investments. Further to that point, most investors seek and deeply desire to get consistent and reliable returns from that stock market. This thus means that a consistent, expected, and non-volatile return is the ideal for most investors; but the stock market does not guarantee this, and for a certainty, will not provide this over the long term, though there may be times when the volatility and the market behave in a manner that appears both reliable and predictable.
In point of fact, it is fair to state, that the more volatile the stock market is, the more dangerous such a market is to investors, for when confidence levels are breached for investors, for whatever reason, this increases the volatility, and the more volatile a given market is, the more extreme it will be in its performance, whether it be up or more likely, down. In general, markets with low volatility have a tendency to be markets that are going up, for the very basic reason, that confident money has a tendency to maintain its status, for it does not fear, and in absence of that fear, it rides along with the prevailing trend. On the other hand, when markets are volatile, because, for instance, of unexpected and unforeseen events, confidence in any stock, can be breached, and once that confidence is breached, all sorts of havoc may ensue, in which the natural state for investors is to seek safe haven, which thereby takes precedence over greed and at times, basic sensibility. This signifies, that those that are in a market in which all is well, and are accustom to the general predictability and reliability of that market, that they not only have a tendency to invest more, because of their confidence in such, but also because of their complacency that such will continue for an unspecified period of time, and thereby they essentially lose their fear of that market ever becoming volatile or dangerous to their investments. So then, when the inevitable happens, and volatility rises considerably for reasons known or unknown, these investors, that have relaxed their normal diligence in their investments, will typically have an overreaction when they come to the newfound belief, that the sky is falling, and this thereby adds to that volatility and helps to accelerate a down market into coming to full fruition.
The bottom line is that markets do rise and fall, and also they are subject to low as well as high volatility; and as to the belief that the market is ever efficient -- this is a belief that is erroneous, because the prices of stocks are forever changing, even when the market is relatively quiescent. So that, a market in which those that are invested in it, do so because of its low volatility and seemingly steady direction, will be very shaken when that future that they have envisioned is upended, for whatever reason, and thereby when the prevailing direction of that market turns down, and the volatility does come roaring back, many of those investors will thereupon add to their own troubles and increase volatility even more, by panicking, in which, suddenly that which appeared to be normal and reasonable, no longer appears that way.