Tues Sotah 15

The Gemara says the reason there is no Nisuchim by Nazir, since it's a sin to pain yourself by refraining from wine.

Tosfos points out that this Gemara holds that there is a sine to be a Nazir even by a Tahor. We find this Shita in many places. Not like Shnuel in the first Perek of Nazir that says he can hold his hair and say "I'll make this nice" to be a Nazir, and we can call it nice, since it's only a sin if the Nazir becomes Tamai.

Although Shmuel compares it to fasting, which the Gemara other places compares it to a Nazir Tahor (since both is initially forbidding himself to pleasures.) We must say that Shmuel only compares it to a Nazir Tamai. (It's not similar to a Tahor, although they're both initially prohibiting something on them) since fasting is much more stringent refraining from pleasures then nazir, since it prohibits all foods from himself. It's only similar to Nazir Tamai that extends extra prohibitions on himself.