Musar for Bava Kamma 182:15

Tosafot raise the question that since the Talmud in Baba Kama 91b, says that one may inflict injury upon oneself, whereas others who inflict injury upon a person are guilty of punishment, and Samuel there states that the kind of injury that is meant is the imposition of fasts upon oneself, we see that Samuel does not hold the opinion that someone who fasts voluntarily is a sinner! The answer given to this apparent contradiction in Samuel's statements is that such a person is called "a sinner." The proof is deduced from the case of the Nazirite as we have stated above. In answer to this apparent contradiction, we have to say that we deal here with a person who both performs a מצוה, by vowing to become a Nazirite and who at the same time commits a sin in subjecting his body to unnecessary painful and therefore harmful experiences. In this particular case the performance of the מצוה outweighs that of the sin committed. We know that fasting is sinful since if someone fasts on the Sabbath as an antidote to a bad dream, the sages rule that since he chose to fast on the Sabbath he must fast on another occasion as a penance, even though the original fast may have annulled the evil decree that he had seen in his dream and which had caused him to fast in the first place.

The example quoted from Baba Kama 91b, may also allow an easy solution. At first glance the Nazir described there who has afflicted himself by denying himself the use of wine may be one who has remained "pure" throughout the currency of his vow, and even such a Nazirite is described as a sinner for having abstained from wine. We have another place in the Talmud (Nazir 19a) where Rabbi Eleazar Hakappor is apparently on record that a "pure" Nazirite is remiss for having made a vow to abstain from wine. After various discussions both there and elsewhere, the Talmud concludes that Rabbi Eliezer Hakappor considers the vow of abstention from wine sinful only if the Nazirite has been remiss in the other conditions he has imposed upon himself, i.e. has defiled himself. Rabbeinu Tam there comments that the statement that the Nazirite is not a "sinner," is to be understood as relative to the מצוה he has performed, but not as an absolute statement. In either case the מצוה of assuming these vows stems from the position of the paragraph of the Nazirite in the Torah immediately after the legislation dealing with the debasement of the סוטה and the statement of the rabbis there that anyone who observes what happened to such a woman has good reason to become a Nazirite in order to fortify himself against the evil urge which might have tempted him to commit infidelities.