Musar for Nazir 37:3

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.