|St. Jerome Writing, Valletta, thanks to Wiki|
Athletes mortify themselves regularly, if they are honest, by being on strict regimes of diet and exercise, as well as practice.
Artists who are real artists and not merely narcissists or propagandists, mortify themselves by discipline to the rules of their particular art.
Many professional mortify themselves, denying short-term ease for long-term professional goals which require hard work and planning
Christians deny even lawful pleasures and goods for the sake of spreading the Gospel, such as missionaries, who endure hardships and strange surroundings in order to obey the command of Christ to preach to all nations.
Sometimes we must do penances for our sins, penances which include mortifications.
However, it is not the external suffering of mortification which indicates saintliness. Even the pagans denied themselves certain pleasures for a good, such as being fit or being a Stoic.
It is only in the intention and the cooperation with grace which causes mortification to be efficacious.
St. Jerome wrote this:
Be on your guard when you begin to mortify your body by abstinence and fasting, lest you imagine yourself to be perfect and a saint; for perfection does not consist in this virtue. It is only a help; a disposition; a means though a fitting one, for the attainment of true perfection.
Mortification leads us to a different perspective about life now and after.
What causes one to mortify one's self are the moral virtues, which are thereby enhanced.
The moral virtues are the Four Cardinal Virtues of Prudence, Temperance, Justice and Fortitude. Here is the CCC on these virtues:
1803 "Whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is gracious, if there is any excellence, if there is anything worthy of praise, think about these things."62
A virtue is an habitual and firm disposition to do the good. It allows the person not only to perform good acts, but to give the best of himself. The virtuous person tends toward the good with all his sensory and spiritual powers; he pursues the good and chooses it in concrete actions.
- The goal of a virtuous life is to become like God.63
1804 Human virtues are firm attitudes, stable dispositions, habitual perfections of intellect and will that govern our actions, order our passions, and guide our conduct according to reason and faith. They make possible ease, self-mastery, and joy in leading a morally good life. The virtuous man is he who freely practices the good.
The moral virtues are acquired by human effort. They are the fruit and seed of morally good acts; they dispose all the powers of the human being for communion with divine love.
1805 Four virtues play a pivotal role and accordingly are called "cardinal"; all the others are grouped around them. They are: prudence, justice, fortitude, and temperance. "If anyone loves righteousness, [Wisdom's] labors are virtues; for she teaches temperance and prudence, justice, and courage."64
We cannot grow in these virtues unless we accept and practice some mortifications. And, this hurts. Mortifications are supposed to hurt.
That is the whole point. The world cannot understand this at all. We who do penance seem crazy to a world bent on pleasure.
Without the goal of perfection and the union with God, none of this makes sense.
But, without a sense of penance, we shall never achieve perfection.
For more insight see the Catholic Encyclopedia articles on mortification and asceticism, from which some of these insights were gleaned.