Mastery is the art of being at cause over a discipline.
The absence of mastery is the presence of mediocrity, and an even lower stage is failure. These two experiences, mediocrity and failure, create low moods, unpleasant life experiences, and consequences of lack, limitation, negativity, and scarcity. In other words, varying degrees of anguish.
Mastery, then, is a decision to excel at something, putting in the time and effort to achieve that excellence, and creating a positive influence on the world.