In this week's parsha, the Torah delineates the laws of sotah - a woman who has acted indecently and is suspected of having committed adultery - and the laws of the nazir - one who has accepted upon himself a temporary "code of holy conduct" including abstention from wine. Rashi (Bamidbar 6, 2), citing Chazal, asks why the Torah juxtaposes these two mitzvos. He answers, that the Torah is implying that one who sees a sotah in her state of degradation should abstain from wine since wine can bring one to adultery.
Rav Wolbe (Shiurei Chumash) comments that we tend to think that witnessing the degradation of a sotah would automatically arouse feelings of repugnance toward her degree of corruption. However, the Torah tells us that our reaction should be more profound. When we see another's transgression, we must take the proper precautions so that we don't end up committing the same misdeed. One might ask, "Why? What does her misconduct have to do with me?" The answer is that what happens to another person can happen to you, too. If he slipped, then it is quite possible that you might slip too. Therefore, the Torah warns us to take the necessary precautions lest we follow the wayward conduct of the sotah.
The first step in preventing such behavior is contemplating what brought this woman to her level of decadence. Generally, such situations don't just happen, rather, they evolve over the course of time. Someone visited her, they had a drink together, and one thing led to another. This behavior must be nipped in the bud which necessitates an abstention from wine, i.e. accepting upon oneself to become a nazir.
Rav Wolbe continues that if this idea it true when one witnesses another's transgressions, all the more so it holds true regarding one's own transgressions. There is generally a process which leads up to a sin, and we must take the proper precautions to ensure that we stop this process in its tracks. If going somewhere, doing something, or talking to somebody, invariably causes us to sin, then we must set up concrete boundaries that prevent this behavior. Abstaining from these activities achieves a high level of kedusha that parallels the kedusha a nazir achieves through his abstention from wine.