Nazir, Chapter 6, Mishnah 5. This mishnah compares the three prohibitions that apply to a nazir, and teaches in which respects each one is more stringent or more lenient than the others.
"Sheloshah minin asurim b'nazir" - three categories are forbidden to a nazir:
"Ha'tumah" - not to become tamei through contact with a corpse.
"V'ha'tiglachas" - not to cut his hair.
"V'ha'yotzei min ha'gefen" - not to eat or drink anything that comes from the grapevine.
"Chomer b'tumah u'v'tiglachas mi'b'yotzei min ha'gefen" - tumah and cutting the hair are more stringent than what comes from the vine:
The reason for this stringency is "she'ha'tumah v'ha'tiglachas sosrim" - one who transgresses them undoes his nezirus and its terms. One who comes into contact with a corpse loses all the days of his nezirus (depending on the type of tumah), and one who cuts his hair must grow it again for thirty days. In contrast, "v'ha'yotzei min ha'gefen eino soser" - one who drinks wine has indeed transgressed his nezirus, but he does not undo any part of it, and the nezirus remains intact.
"Chomer b'yotzei min ha'gefen mi'b'tumah u'v'tiglachas" - what comes from the vine is more stringent than tumah and cutting the hair:
The stringency of what comes from the vine is "she'ha'yotzei min ha'gefen lo hutar mi'klalo" - matters of the grapevine never become permitted in a general way, not even for the sake of a mitzvah. For example, one who took an oath that he would drink wine and afterward took a vow of nezirus is not permitted to drink that wine, even though he has a mitzvah to drink it on account of his oath. And all the more so is he forbidden regarding the mitzvos of Kiddush and Havdalah, which are only rabbinic.
Regarding tumah and cutting the hair, however, there is a leniency, "she'hutru mi'klalan" - at times they do become permitted:
The shaving of a mitzvah: a nazir who became a metzora - a metzora, upon becoming purified from his tzaraas, is obligated to shave off all his hair. The mitzvah to shave is a positive commandment, while the prohibition against shaving for a nazir is a negative commandment, and we have a rule that "a positive commandment overrides a negative commandment." Therefore he shaves his hair in this situation.
A meis mitzvah: a nazir does not come into contact with a corpse, not even that of his relatives. But a meis mitzvah, for whom there is no one else to care, may be attended to even by a nazir, who becomes tamei for it and is obligated to care for it.
It emerges that what comes from the vine carries the stringency that it never becomes permitted, unlike tumah and cutting the hair, which under certain circumstances do become permitted.
"Chomer b'tumah mi'b'tiglachas" - tumah is more stringent than cutting the hair:
Even between tumah and cutting the hair themselves there is a difference, and tumah is more stringent than cutting the hair in two respects:
"She'ha'tumah soseres es ha'kol" - one who becomes tamei forfeits all the days of his nezirus, whereas cutting the hair "einah soseres ela sheloshim yom" - it is enough for him to grow his hair for another thirty days.
"V'chayav b'havaas korban" - one who became tamei brings a korban on account of his tumah, whereas one who cut his hair "ein chayav b'havaas korban" - is not obligated to bring a korban.
In summary: in this mishnah we compared the three prohibitions that apply to a nazir: tumah, cutting the hair, and what comes from the vine. Tumah and cutting the hair are more stringent in that they undo the nezirus, while what comes from the vine does not undo it; what comes from the vine is more stringent in that it is never permitted, not even for the sake of a mitzvah, while tumah and cutting the hair are permitted in the case of the shaving of a mitzvah and a meis mitzvah; and between those two, tumah is more stringent than cutting the hair, since it undoes everything and requires a korban.