Ish o ishah ki yafli l’ndor neder nazir ... if a man or woman shall take upon themselves a vow of nezirus (Bamidbar 6:2)
Rashi, quoting the Midrash, comments that the parashah of the nazir is juxtaposed to the parashah of sotah - the woman suspected of adultery - to teach us that one who sees a sotah in her state of disgrace [i.e., after she drinks the waters which prove her guilt] should swear off wine [i.e., accept the vow of nezirus], for it is wine that brought her to commit adultery.
The comment is somewhat difficult to understand, for the status of the nazir is enigmatic to say the least. On the one hand he [or she] is referred to as a kadosh - a holy or sanctified person - by virtue of his having undertaken a vow of isolation from the community. He is not to shave to demonstrate that he has no regard for social conventions. He is forbidden to come into contact with tumah to show that he is seeking a more spiritual kind of existence and desires kirvas Elokim - closeness to G-d - which is unattainable when one deals with ritual impurity [tumah]. He foreswears the fruit of the vine, for wine can be a substitute for spirituality [see commentary of Netziv to Shir ha-Shirim].
On the other hand, the nazir, at the end of his period of nezirus, must bring a korban chatos - a sin offering - for the Torah is critical of his having given up on society and on the pleasures that G-d has granted man in this world. Celibacy and living like a monk are not Jewish values, for G-d has commanded us to elevate the material into the spiritual. We recite a berachah before eating to show that we recognize that everything has G-dliness in it. We recite asher yatzar when we relieve ourselves to indicate that even the most mundane acts can have spiritual meaning.
My rebbi once explained that one can be both a kadosh - a holy person - and a choteh - a sinner - at the same time. The nazir, he explained, recognizes his weakness. He knows that unlike others, he does not have the fortitude or strength to resist sin. He therefore separates himself, sacrificing some of the good that G-d intended him. True this is a sin, but it comes from an honest, albeit mistaken personal assessment and is therefore atoneable. Every person can avoid sin, no matter how weak he thinks he is. Note that the nazir brings a chatos - a sacrifice that is offered for sins committed b’shogeg - inadvertently. Had accepting nezirus been a true sin, there would be no atonement, for sins committed b’meizid - with full intent - have no korban.
The Midrash comments that one who sees the sotah in her state of disgrace should foreswear wine. Note that the Midrash refers to her state of disgrace. If anything, the miraculous process of the sotah should be a source of strengthening a person. We have a situation where a man suspects his wife of adultery but has no means of proving whether or not she is guilty. The trust between them is tenuous at best and he therefore brings her to the kohen who is the only person who can determine the truth. Based on the results of her drinking the potion that the kohen provides, we will find out what really happened. If she is innocent then she will be blessed with children. But if she is guilty, her body will explode and all will know of her infidelity.
People are standing by, waiting to see what will transpire. The entire ceremony is degrading, especially if she is innocent of the charge. This is the point of sotah b’kilkulah - the sotah at her time of disgrace; not later when her stomach and loins have collapsed, but now when she stands before the kohen. Only she knows whether or not she has committed adultery. She can stop the entire process simply by admitting the truth. She will not have to drink the potion, she will not be subject to the death penalty because there are no witnesses. Her husband will divorce her, but she will remain alive.
But she says nothing, somehow believing that nothing will happen to her. One bystander, more sensitive than most, is cognizant of his own weakness. He understands how hard it is not to sin, he knows how weak he is. The others present see only the miracle. He sees how sin - or even the suspicion of sin - degrades man and he therefore decides that he can only save himself by accepting nezirus. Wine can bring man close to G-d by releasing the inhibitions that often prevent him from expressing his spirituality. But that same wine can also serve as a cover for the truth and it takes a kadosh - a person who is inherently holy - to realize what it does to him.