An overconfidence, self-satisfaction, or smugness that is bred from success and makes us unaware of potential dangers or threats, which is what is meant by complacency behavior. 

16 minutes ago, drpaladin said:

You never quite know what you'll find or where it will end up.

Exactly how I feel, and particularly in terms of where it will end up.

27 minutes ago, Bill W said:

Complacency is driving home from work, or to work, and not being able to remember anything about the drive.  Going to and from the same place numerous times tends to make you complacent and you get there an autopilot.  

The saying is that familiarity breeds contempt. In situations like driving and working familiarity does breed complacency. Although as Churchill commented, "without a certain degree of familiarity we could not breed anything".

Complacency has lost more battles than Custer and France combined.

8 hours ago, Bill W said:

Complacency is driving home from work, or to work, and not being able to remember anything about the drive.  Going to and from the same place numerous times tends to make you complacent and you get there an autopilot.  

I used to be able to drive on autopilot all the time. I could even be on autopilot on never driven before roads, avenues, and highways, which is scary if you think about it too long. I still do to some extent but take medication to help me stay aware and alive. Mental illness is not a fun disease.