We sometimes do things because our Yetzer Hara, our desire, pushes us to act without seriously and honestly considering the consequences. The Torah tells us that it is good to create a bubble in time, a period during which you remind yourself that you do have will-power and you can resist your desires. Wine and grapes are very enjoyable, but you will not touch them for thirty days. Certainly, when you drink wine your inhibitions are diminished, and it is important to be vigilant for those thirty days. You are trying to strengthen your ability to withstand desires, and drinking wine is the worst thing you can do.
Another enemy of spiritual growth is vanity. Physical vanity focuses one's attention on himself to the detriment of others, and it inflames all physical desires. Also, a person who is conscious of his beauty might not put as much single minded effort into the study of Torah and Mussar. Vanity can impede growth in Torah, in middos, and in chesed. So the Torah says, grow your hair for thirty days, and perhaps you will look unkempt during that time, or, if you are lucky enough to have beautiful hair, by the end of thirty days you will have brightly shining curls- then go and cut them off and burn them in the Beis HaMikdash. Unlike Savonarola's foolish exhibition, this is a true bonfire of the vanities.
Perhaps the greatest enemy of spiritual and Torah growth is despair. So many people have given up on themselves! They don't always say it in so many words, but you see them all the time. They are bitter and unsympathetic, they sit there with the slichos and just stare off into the distance. They have decided that nothing they've tried has worked, they are failures in Ruchniyus and Gashmiyus, they are just losers. All they can do is go through the motions, because they are never going to get any better than they are. In the Slichos for Tzom Gedaliah it says
טכסת מקדם אלו ימים עשרה, יחיד בם לשוב ולמצוא כפרה, כל השנה כולה לרבים מסורה, לשוע ולענות בכל עת צוקה וצרה, מהר היחיד ושב בינתיים מוחלין לו, נואש ולא שב אין תקנה לעוולו, סדר וערך כל אילי נביות להועילו, עותר וצועק ואין שומע לוDespair is a terrible problem. We feel bad for a person that despairs, but from the Pizmon we see that sometimes it's just an excuse- it's the way a person avoids doing what he really knows he can do. Sometimes, it practically paralyzes a person's initiative and saps his energy. Whatever it is, it is a terrible weapon of the yetzer hara. Seeing the dead causes greater despair- that person can't do anything any more, oh, what's the point in trying! But the truth is that as long as you're alive, you can grow, you can change, you can make a difference. All that you need is life and a decision to try, even if only to try something small. The only person that cannot do teshuva is the בן סורר ומורה, and a בן סורר ומורה never existed and never will exist. To help deal with the self-destructive trait of despair, the Nazir is told to spend a period of time focused on life. You are alive- you have been given the gift of life. Do something with the time you have! It's not too late for you.
תאוה, גאוה, ויאוש
Nezirus addresses the problems of תאוה and גאוה and יאוש. The Torah tells a person to carve out a period of time during which he reminds himself of what is possible and what is worthy. Thirty days really is not long enough to permanently change who you are, but it is a good way to remind yourself of what is important, and what you are capable of doing.
Every year, people come over to me and say that this whole business of acting different during Ellul, of the Aseres Yemei Teshuva, it's just hypocritical. We accept chumros (OC 603,) we avoid Pas Palter or we're makpid on Chalav Yisrael, we don't talk in shul or speak lashon hara. But come the eleventh of Tishrei, all the chumros go out the window. What's the point? To them I say, look at the parsha of Nazir. Obviously, the idea of Nazir is not to say, OK, I'm on the wagon for thirty days, but on day thirty one I'll be flat on my back in the cemetery with a bottle of Thunderbird. The idea is to create a bubble of time during which you make a conscious attempt to remind yourself of what is important, of what is worthy, of what you are capable of doing. You probably won't see enormous changes, but the Parsha of Nazir tells us that something is likely to remain, and you will come out a better person.