For me, it's a two-fold answer. I'm terrified of being unprepared for a job - not having enough lights, not having the right lights, not being able to humanly cover, combat and overcome every challenge that is going to arise from the moment I arrive at a location to the moment we wrap (client liasing, power issues, equipment problems, etc.)
On the other hand, I've come to realize from forums like this, from ASC magazine, etc. that I can only prepare as much as I can to be ready. And once we're on set, things WILL inevitable happen that aren't expected. And half of my job seems like it's about dealing with those things as they come up. I guess that's the fun of it.