Self-Control Is A Resource

And one that is easily depleted:

[N]o matter what the self-help books say, the research suggests that willpower isn’t a skill. If it was, there would be some consistency from one task to the next. Instead, every time you exert control over the giant system that is you, that control gets weaker. If you hold back laughter in a church or classroom, every subsequent silly notion is that much funnier until you run the risk of bursting into snorts.