The Rambam is famous for adopting the negative Talmudic view of the nazir, who voluntarily abstains from some worldly pleasures. However, on closer look, the Rambam actually views such asceticism positively, neatly fitting into his understanding of holiness.
I. Enjoy Life
The nazir is called kadosh, holy (Num. 6:8). But on the other hand, he must bring a sin-offering at the end of his nazir period. In the Talmud (Nedarim 9b-10a), Shimon Ha-Tzadik believes that a nazir who becomes impure and has to extend his period of abstention is consider a sinner, because he surely regrets the length of time. R. Elazar Ha-Kapar argues that even a regular nazir is a sinner for abstaining from the pleasure of wine (and even more so someone who fasts). But others see the nazir as an ascetic hero.
The traditional understanding, for example seen readily in Prof. Nehama Leibowitz’s Studies in Bamidbar (pp. 55-56), is that Rambam adopts the view that the nazir‘s sin is his abstention from worldly pleasures. Rambam (Introduction to Avos, ch. 4; Mishneh Torah, Hilkhos De’os 3:1) famously advocates a middle path. We should not veer to any extreme but should instead adopt a moderate approach. The nazir must temporarily take extreme measures to rid himself of negative traits but he must return to the middle path after his cleansing routine. The sin-offering is for his extremism, for his abstinence.
II. Don’t Enjoy Life
Ramban (Commentary to Num. 6:14) embraces the nazir‘s asceticism. Why must he bring a sin-offering? For leaving the nazir regimen, for abandoning a holy, ascetic life and returning to worldly pleasures. The nazir is currently in an ideal state and must bring a sin-offering for his return to normal, less-than-ideal life.
It appears that the Rambam advocates a life of moderate partaking in worldly pleasures while Ramban sees asceticism as an ideal. However, this is only partly true. In fact, the Rambam plainly contradicts this thesis (at least) three times in his Moreh Nevukhim (3:8, 38, 54). To grow intellectually and spiritually, Rambam requires a scholar to distance himself from worldly pleasures. Food, drink and other physical delights should be partaken in the bare minimum, to maintain focus on what is truly important. Rather than advocate a middle path, in Moreh Nevukhim Rambam advocates the extreme path of asceticism.
III. The Holy Life
R. Yonasan Blass, in his Mi-Nofes Tzuf (vol. 2, p. 836-837), explains that Rambam’s middle path, which he advocates in his Commentary to the Mishnah and his Mishneh Torah, is the proper approach for the layman. The average, and even above-average, Jew should live a balanced life. However, he recommended a different life for the exceptional scholar, the intellectual and spiritual leaders. To reach greatness, according to the Rambam, the middle path will not do. You need to adopt an ascetic regimen and focus on the esoteric but all-important issues. Dr. David Shatz (“Maimonides’ Moral Theory” in The Cambridge Companion to Maimonides, pp. 180-181) quotes academic sources that explain the Rambam similarly:
Clearly the middle way has been rejected except for the ordinary run of individuals… We can surmise that the problem of asceticism arises when it is pursued by those who are unprepared for it and still have sensual urges. People who have overcome these urges are at a higher level, and for them asceticism is the proper course.
It seems Rambam and Ramban do not disagree as much about this as had been previously thought. Their approaches fit in well with their understandings of a different subject. We have suggested elsewhere that according to the Rambam, holiness comes from refraining from what is bad (link). While according to Ramban, holiness comes from going beyond the required.
In the case of the nazir, these different views coincide. According to these two scholars, enjoying worldly pleasures is permissible but ultimately bad for you. Therefore, according to Rambam, refraining from worldly pleasures is avoiding something bad, which yields holiness. According to Ramban, it is taking on extra measures, refraining from that which is technically permissible. Therefore, he also agrees that it yields holiness. The bad but permissible is the meeting place between these two visions of holiness.