Discipline should not be practiced like a rule imposed on oneself from the outside, but that it becomes an expression of one’s own will; that it is felt as pleasant, and that one slowly accustoms oneself to a kind of behavior which one would eventually miss, if one stopped practicing it.

Erich Fromm (1900-1980) American psychoanalyst and social philosopher

The Art of Loving, ch. 4 (1956)