If a person commits a sin and repeats it, it appears to him as permissible (Yoma 86b).
As every scientist knows, different substances have different properties. Some liquids freeze at 0 degrees C; others at minus 60 degrees C. Some materials burn at higher temperatures than others, and some metals have greater resilience than others. In order to know how to work with any substance, we must know what its particular properties are. Ignorance of a substance's properties results in failure of the project at best and disaster at worst, as in the case of an engineer who overestimates the strength of the cables that suspend a bridge.
What are the properties of a human being? Physically, we know that we can survive only within a certain range of temperatures. But what about the guidelines for our spiritual survival? It would be foolish to think that there are no limits. Excellent guidelines do exist, and these are available in Jewish works on ethics.
The above Talmudic passage is an example. A person knows that doing something is wrong, but submits to temptation and does it anyway. He or she is likely to feel guilty, do teshuvah and thereby avoid repeating the act. However, if he or she fails to do so and repeats the forbidden act, the stimulus necessary for teshuvah may be lost. The Talmudic authors were astute students of human behavior, and they tell us that two consecutive commissions of a wrong act may cause people to totally lose their perspective; they are now apt to develop an attitude whereby what was once wrong is now perfectly permissible.
We do not have much leeway. If we do not promptly try to amend a wrong act, we may lose the opportunity to do so, because if we repeat it a second time, we may no longer realize that it is wrong.