There is one main reason why complacency is bad: selloffs. Because as Bank of America explains, in an environment in which there are "too few bears", and where investors are "not prepared for a downside correction", when you do finally get a sell off for whatever reason, with nobody hedged and otherwise prepared for such an outcome, the only logical continuation is piling on until one gets selling exhaustion. And in a world in which hedge fund leverage is about 500%, by the time exhaustion comes, there will be very few left standing.
From Bank of America:
Sentiment an equity market risk: II % Bears dropped to 14.1% = too few bears
Even with the increased volatility since mid-January, there are still too few bears out there based on Investors Intelligence (II) % Bears. The most recent survey reading as of January 24 was 15.3% vs. 15.1% the prior week. This is complacent and contrarian bearish from a sentiment standpoint. With II % Bulls moving down to 53.1% from 57.6%, the bulls did reign in their horns, but not enough to move II % Correction to a contrarian bullish level that would suggest that too many investors are looking for a correction. As of December 20, 2013, Investors Intelligence (II) % Bears extended deep into contrarian bearish territory below the 20% level hitting a 14.1% - the lowest level for II % Bears since March 1987. II % Bulls moved up to 61.6% as of December 27, 2013 - the highest level since October 2007. Sentiment is an equity market risk and confirms the complacent readings for the 5-day put/call ratios .
Put-call ratios are complacent & contrarian bearish
Both the 5-day and 25-day CBOE Total put/call ratios are overbought and contrarian bearish. The 5-day total put/call ratio reached the lowest level since early 2010 while the 25-day total put/call reached the lowest level since early late 2004. These put/call ratios are at levels that suggest investors are not prepared for a downside correction. In terms of market sentiment, this is contrarian bearish.