Musar for Berakhot 62:25

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.