Self-love, which is good, can be perverted to self-adoration, in which one says, ” I am my own law, my own truth, my own standard. Nobody can tell me anything. Whatever I call right is right; what I call wrong is wrong. Therefore, I am God.” This is the sin of pride, the perversion of self-love into egotism. Such undue self-inflation is one of the principal causes of unhappiness; the more a balloon is inflated, the easier it is to puncture it. The egotist walks warily, in constant danger of having his false values destroyed.
The sex instinct, which is good, can also be perverted. In the days of pagan Rome some corrupt individuals attended banquets, gorged themselves on food, tickled their throats, disgorged the food, and then went back and ate more. This was wrong because, as reason told them, one eats to live and the pleasure must not be separated from its function. In like manner, when the fires of life are aroused deliberately, not to light new torches of life, but to scorch the flesh, there is the sin of lust.
Third, man’s legitimate desire for self-expression through ownership can be perverted into an inordinate passion for wealth, without regard for either its social use or the needs of the neighbor. This is the sin of avarice, in which a man does not possess a fortune, but a fortune possesses him.
Because man’s will can pervert the good passion, instincts, cravings, and aspirations of man into pride, lust, avarice, the Church enjoins mortification—through prayer, which humbles the proud soul, through fasting, which harnesses the errant impulses of the body, and through alms, which detach us from inordinate love of things. In the higher realm, the Church allows some chosen souls to take the vow of obedience to atone for the proud, the vow of chastity to redeem the licentious, and the vow of poverty to compensate for the greedy.
—from Peace of Soul by Venerable Fulton Sheen, Ch. 8: Sex and Love of God