If you feel that you are the only person capable of doing a job that the future of humanity depends on -- it isn't really a choice.

Even on a slightly less dire level -- CEOs have their employees and employees' families fate resting on their shoulders. It isn't easy to trust someone else to do it right. You can't just take a month off and hope everything goes ok.

When you have a moral imperative to work it isn't so easy as taking a vacation anytime you want.