The gemoros N'dorim and Shvuos are replete with the rule that anything that is already binding by the oath of acceptance of the Torah cannot have an additional oath added. It is astounding to have someone voluntarily separate himself from the pleasures of this physical world. Perhaps there is a symbolic message here. Correspondingly, the Torah has given us 613 mitzvos which include 365 restrictions and 248 positive precepts. The nozir has taken upon himself something extra, beyond the restrictions of the Torah, and has included them in the list of restrictions.
Human hair is in some ways beyond the basic body.
We find that it can be cut with feeling no pain, that although a person has died, it continues to grow for a short while, and that the prohibition to not derive benefit from a human body after the person has died does not apply to hair. All these points show us a beyond the body relationship of hair with the rest of the person. Since the nozir has taken extra restrictions upon himself, the Torah wants him to be continuously conscious of this, and gives his hair sanctity that it would not otherwise have. The Torah gives us a list of relatives to whom the nozir may not defile himself. The medrash says that the bnei Yisroel are called Hashem's sister when they fulfill the mitzvos of circumcision and bringing the Paschal lamb.