Among the laws which one who becomes a Nazir voluntarily accepts upon himself as a result of his vow to be a Nazir is that he will hold himself back from drinking wine or eating any grape items and from cutting his hair. Why should someone want to become a Nazir in the first place? For some, it may be due to a bad experience brought on by the harmful effects of wine. For others, it may be that one might want to stay away from certain physical desires and avoid physical pleasures in order to better concentrate on keeping the Mitzvos of the Torah. In any case, one's goal in becoming a Nazir is to change himself and his level of Kedushah, and to become better person.
The question may be asked, however, if all this is an indication that the Torah believes that one should preferably withdraw from the pleasures of life. The answer to this is no, because the world and its resources were created for man's benefit. To constantly deny oneself life's pleasures is to imply that the world is not good enough, and this denigrates Hashem's gifts. In certain other religions, the holy people are expected not to marry and to avoid all the pleasures of this world. But this is not the Jewish view. In our religion, we are supposed to benefit from what Hashem's world has to offer, but only according to the Torah's guidelines. One who has trouble for some reason following those guidelines may have to become a Nazir and temporarily abstain from certain pleasures of the world in order to improve his character and his relationship with Hashem.
With this idea, we can understand some of the restrictions accepted by the Nazir. When a person drinks even an average amount of wine, he loses clarity of mind. This may lead to immoral misbehavior. A Nazir is also prohibited to eat grapes and raisins, including their seeds. If he would approach a vineyard, he would be warned not to walk through there and to take a detour. This shows that sometimes, in order to achieve Kedushah it is necessary to go to an unusual extreme. Why is a Nazir not permitted to cut his hair? The purpose of a haircut is to beautify one's appearance. Too much emphasis on physical beauty, however, can lead a person to sin. By letting the hair grow long, and then, at the end of one's period of being a Nazir, shaving it completely (as required), would banish from one's mind the thoughts of beautifying the body for the wrong reasons. Again, we see that sometimes it is necessary to go to an extreme in order to improve one's overall character and better be able to live the life Hashem wants us to live.