Q. Why is it important to avoid sinful situations?
A. For the same reason you avoid infectious situations. What infectious situations are to the body, sinful situations are to the soul.
Both body and soul have ideal conditions when they are both in perfect working order. In the body we refer to this as “health“; in the soul we refer to this as “virtue” (or “grace” if you want to speak supernaturally). Both body and soul expend huge amounts of effort to reach and maintain this perfect working order. Left to themselves, they both tend in the opposite direction. (Dead bodies float downstream, only live ones – virtue, health – can swim upstream against the current.)
Some things we do only slightly injure the health of the body e.g. a graze, a cut, getting drunk, eating some rotten food, etc. The body quickly recovers from these anomalies. However, there are others that are incompatible not just with the healthy life of the body, but with the very life of the body e.g. an open heart, a leaking brain, a severed neck, a punctured artery.
Similarly, there are things that we do that only slightly injure the virtuous life of the soul e.g. waking up late due to laziness, cheating in a game or sport, entertaining some vain thought, keeping others waiting due to lateness, etc. The soul quickly recovers from these anomalies. However, there are others that are incompatible not just with the virtuous life of the soul but with the very life of the soul e.g. masturbation, missing Church on Sunday, corruption, getting drunk, etc.
Acts injurious to the health of the body are generally called minor illnesses; those that are incompatible are called terminal illnesses.
Acts injurious to the virtue of the soul are called venial sins; those that are incompatible are called mortal sins (or “deadly” depending on what translation of St. John’s letters you’re using).
Why avoid sinful situations? Because in the same way you avoid infectious situations to preserve your body’s healthy life, you should also avoid sinful situations to preserve your soul’s virtuous life.