Common wisdom suggests complacency is built up over time working a process over and over. “Habituation” may take over. This means you go on autopilot and your ability to notice changes in the hazard, or perhaps your own behavior, fades.
When this happens our behavior drifts toward risk – because the hazards are not as salient to us. Since we notice them less, these hazards no longer influence us to take extra precautions for safety. In fact, we probably don’t see the hazards as hazardous any more.