Rabbi Elozar HaKappar asks: What does the verse mean when it says, “and he shall atone for him for having sinned on his soul?” What “soul” did he “sin” against? It must be referring to the fact that he pained himself by abstaining from wine. This additionally teaches us that if this person who merely abstained from wine is called a sinner, someone who abstains from many things is certainly a sinner.
Ben Yehoyadah explains why one who deprives himself from wine or any food is regarded as a sinner. Portions of one’s soul are contained within foods and drinks. When one recites a blessing before eating these foods, he can cause a remedy for those parts of the soul, and through his blessing, they will be able to go to their rightful place. It emerges that one who declares himself to be a nazir and therefore refrains from eating grapes or drinking wine, is sinning regarding his soul, for now his soul will remain deficient.
Furthermore, there are many mitzvos where wine is required, such as kiddush on Shabbos and Yom Tov, havdalah, birkas hamazon, bris milah and sheva brochos. Chazal established the mitzvos in this manner in order to rectify the sin of Adam Harishon, which was with wine. One who vows to be a nazir and therefore abstains from drinking wine causes anguish to his soul.