A “sin” is determined by whether or not you ask the Creator for the strength to rise above your sensations of “good” and “evil.” The first time you do something is not a sin! Maybe you have not yet discerned good and evil and your attitude to Him. However, if you do it a second time, it becomes a sin. And if you repeat it a third time, you descend from your degree; you cannot remain on it any longer.
With the first interaction of the pleasure and the desire, a desire for the pleasure (a vessel, Kli) is created. At the second interaction of the pleasure and the desire, one receives a feeling of transgression (a sin), reception for one’s own sake. But if one receives pleasure in the desire for a third time, then it not only indicates the performance of a sin, but also shows that the sensation of the sin was not sufficient in order to refrain from it.
This can be likened to a situation where the sentence ruled by a court is weaker than the criminal desire and, therefore, does not reform one from stealing in the future. And then a decision is made to conceal me in the prison of this world.
From the 1st part of the Daily Kabbalah Lesson 9/14/10, “Preparation for the Reception of Torah”