Will-power is a finite resource in the day

The scientific name for this is “ego depletion”, but recent evidence is suggesting that it may not actually be a thing, with multiple attempts to replicate the original result failing to do so.

In fact, the best evidence for ego depletion seems to be possibly that it’s a sort of nocebo. That if you believe your willpower is spent for the day, you give yourself an additional excuse not to make harder choices.