B'midbar chapter 6 explains the laws of one who vows to become a nazir, but I've long been puzzled by why one would do this. (Our most famous nazir was commanded in this; Shimshon didn't make that choice.) What is the benefit? Was this just put here as a way to channel people looking to take on stringencies, or does it serve some other purpose?
The gemara in Nedarim 10a says:
Perishus (separation from the world) is a very commendable thing. There's a whole 'gate' (section) in Duties of the Heart which discusses the reasons for this.
As to why the Talmud frowns on the Nazir for forbidding things which are permitted, I read from Rabbi Uziel Milevsky's book on the parsha in Nasso that this refers only to one who has the mistaken belief that the pleasures of the world are "bad". But for one who takes on Perishus to help him in the service of God, this is something commendable.
"When a man turns [excessively] to the physical pleasures, he will not stop nor rest day and night, and all of his aspirations will be to fulfill his desires. And all of his acts are not l'shem shamayim (for G-d), because he will always look first - if he will receive benefit from this - then he will do it. But if not, - he will drop it all" — from end of Gate #1 of Duties of the Heart's Marpe Lenefesh commentary. See Gate #9 for much more.
"The yetzer hara comes primarily through eating and simcha (parties). (ein hay'r ba ela mitoch achila v'simcha)" vilna gaon on Mishlei-7:14 - this is exactly what the nazir is trying to avoid. Imagine a college student, who makes a vow against alcohol, how much of the mad drunken parties he will be saving himself from.