I think part of it stems from faulty reasoning. If you believe the lights could go out, then they will.

If you don't feel like carrying one, you rationalize your unpreparedness by thinking, ah, the lights won't go out but if they do, they'll be back on in a jiffy.

If you carry a flashlight, then you believe the lights may go out. If you don't carry one, then you don't think the lights will go out, but that person who is carrying one believes in the frailty of the system.

People don't like seeing the system's potential shakiness manifested by anyone's being prepared. To them, it seems as though they are unprepared and that makes them nervous, which they act upon by joking about or making light of the prepared person.

I don't know if I've explained my thoughts clearly here, but I sure have experience with that sort of mindset. Grin.