The amount of will-power or self-regulatory strength that is used up when making choices depends on the type of choice being made. Autonomous choices don't result in ego-depletion. If you're doing things you want to do, things you like doing, your brain doesn't have to expend 'will-power'.

Choice and Ego-Depletion: The Moderating Role of Autonomy

The self-regulatory strength model maintains that all acts of self-regulation, self-control, and choice result in a state of fatigue called ego-depletion. Self-determination theory differentiates between autonomous regulation and controlled regulation. Because making decisions represents one instance of self-regulation, the authors also differentiate between autonomous choice and controlled choice. Three experiments support the hypothesis that whereas conditions representing controlled choice would be egodepleting, conditions that represented autonomous choice would not. In Experiment 3, the authors found significant mediation by perceived self-determination of the relation between the choice condition (autonomous vs. controlled) and ego-depletion as measured by performance.