We get into a rhythm with planes we commonly fly. This can be both good and bad. Good in that the "to do" list comes naturally from experience, and the checklist (check that we did list) becomes memory too. Then we get even more relaxed flying the plane again, as everything "fits". This will then extend to a class of aircraft, which are all similar. So far, it sounds like a good set of circumstances - no "Swiss cheese" holes lining up, for us to fall through, and have an accident.
But then, we allow ourselves to become more distracted than ideal by something outside the aircraft, (a Swiss cheese hole - possibly), then something else does not go quite the way we planned (like a landing gear indicator fails to illuminate when it should). Now, our attention is multiply reduced. Now's the time to pull out the paper checklist, even for an aircraft with which we are familiar, and double check what we are doing.
Because, no one plans to have an accident! When I finally do, I want to be walking away from the bent plane, with the checklist in hand, saying "well at least I did not forget XXX!".
It's up to is to prevent unintended events. Sometimes that's done by reverting to the basics - which certainly includes taking a bit of extra time, and reading the paper checklist. It's your job as the proficient pilot to know when those times are - before you miss the important thing!
I was thinking all this yesterday and today, as I checked myself out in the taildragger amphibian for the first time this season....