Rashi on Numbers 6:2/רש"י על במדבר ו׳:ב׳

1כי יפלא means, IF HE CLEARLY UTTERS (cf. Rashi on Leviticus 22:21 and Note thereon). — Why is the section dealing with the Nazarite placed in juxtaposition to the section dealing with the סוטה? To tell you that he who has once seen a סוטה in her disgrace should abstain from wine, because it may lead to adultery (Sotah 2a).

2נדר נזיר — The term נזר wherever it occurs denotes “keeping aloof from something” (cf. Rashi on Leviticus 22:2). Here, too, it means that he keeps away from wine (Sifrei Bamidbar 22).

3להזיר לה׳ means, TO KEEP HIMSELF ALOOF from wine FOR THE SAKE OF HEAVEN (God).