I say this because volatility is an emergent process. It is the unintended effect of thousands of individual actions which are taken for different reasons. Low volatility is in fact a sign that investors disagree. Prices are relatively stable because buyers can find sellers and vice versa without prices moving much. High volatility, on the other hand, would be a sign of consensus; prices fall a lot when most investors want to sell and rise a lot when most want to buy. With hindsight, stock markets were most complacent at the peak of the tech bubble in 1999, and volatility then was high.
All this might sound trivial. But it has an important implication. It implies that low volatility needn't be a sign of complacency but of the opposite. It might mean that there's a significant fraction of investors who are nervous.