What’s true for an automobile is, in this sense, also true of the human soul.
Theologians have long attempted to explain humanity’s tendency to veer off course: one big sin (that of our first parents in the garden) and it’s almost impossible to go straight without constant correction. Keeping in mind that the New Testament word for sin is hamartia, a Greek word that literally means to miss the mark or to veer off course, we might say that after original sin it’s nearly impossible to stay on the “straight and narrow.”Theologians call this tendency to sin “concupiscence.” The word concupiscence is defined as a strong desire, a tendency or attraction, usually arising from lust or sensual desires. It is, morally speaking, the tendency to go off course. Concupiscence is understood as an effect of original sin that remains after baptism. The waters of baptism cleanse us of original sin itself, but concupiscence remains as a lingering effect. The Catechism of the Catholic Church teaches that “certain temporal consequences of sin remain in the baptized, such as suffering, illness, death … as well as an inclination to sin that Tradition calls concupiscence”
1264 Yet certain temporal consequences of sin remain in the baptized, such as suffering, illness, death, and such frailties inherent in life as weaknesses of character, and so on, as well as an inclination to sin that Tradition calls concupiscence, or metaphorically, "the tinder for sin" (fomes peccati); since concupiscence "is left for us to wrestle with, it cannot harm those who do not consent but manfully resist it by the grace of Jesus Christ." Indeed, "an athlete is not crowned unless he competes according to the rules."
To use another analogy, medical research cautions that a severe sunburn early in life will render a person more susceptible to dangerous skin cancer throughout life. That early sunburn may heal fairly quickly, but its effects last through life, increasing vulnerability to cancer. Precautions must be taken to shield the skin from the damaging effects of the sun’s radiation, since there is a greater susceptibility to skin damage after that major sunburn.
St. Thomas Aquinas taught clearly that concupiscence is a consequence of original sin. Once human beings made the decision to be unbound from the will of God, the harmony within human nature also became unbound. Desires and appetites were no longer in harmony with the intellect or reason, and the two — desire and reason — fought against one another. St. Paul understood this, and described it in his Letter to the Romans: “I see in my members another principle at war with the law of my mind, taking me captive to the law of sin that dwells in my members” (7:23). As a result, St. Paul could write, “I do not do the good I want, but I do the evil I do not want” (Rom 7:19). Even Jesus observed concupiscence in action when He said, “The spirit is willing, but the flesh is weak” (Mt 26:41; see also Mk 14:38). The prophets of the Old Testament understood this interior tension. Jeremiah asked the piercing question, “More tortuous than anything is the human heart, beyond remedy; who can understand it?” (Jer 17:9). Jeremiah understood human nature and spoke often of the stubbornness of their evil hearts (see 3:17 and many other passages), “evil thoughts” (4:14, RSV), and humanity’s “stubborn and rebellious heart” (5:23, RSV). The psalms of David offer lament for sins committed as well as penetrating insight into the lived dichotomy between weakness and grace, the lusts of the flesh and the longing for holiness. “Sin directs the heart of the wicked man; his eyes are closed to the fear of God” (Ps 36:2). In a plaintive cry for God’s mercy, the psalmist acknowledges the dueling desires within him, and acknowledges, “I have been mortally afflicted since youth” (Ps 88:16).