Monotonous tasks require self-control because they interfere with endogenous reward.

Self-control is a necessary component of subjective effort, but it depends only on farsighted motivation, with no additional, depletable resource. The aversiveness of boring tasks probably comes from their interference with endogenous reward, a new and potentially controversial concept. The self-control needed to stick with any kind of aversive experience increases as the values of the competing motives draw closer together.