Ramban answers that a korban chatas is required because the nazir’s return to everyday life is a step down from the spiritual heights he reached during his nezirus. A return to the everyday world is a surrender to the drives and desires that the nazir avoided while in his higher spiritual state of nezirus.
Rabeinu Bachyei disagrees. We never find, he writes, a korban chatas offered for future sins that have not yet been committed; a chatas is only brought after one has already sinned. When the nazir steps back into the mundane everyday routine that may include drinking wine, taking a haircut, etc., he has not yet done anything wrong.
The korbanos of the nazir, explains R’ Bachyei, are not brought because of any sin (he does not say it explicitly, but it seems that this chatas is an exception to the norm because it is brought together with the olah and shelamim). The root of the word korban is k-r-v, to draw closer; the purpose of the nazir’s korbanos is to draw him closer to the source of his spiritual energy, to give him a final boost that he can carry into his regular day to day. “V’achar yishteh ha’nazir ya’yin” – the Torah ends the parsha by telling us that the nazir should return to his everyday life of the past. The purpose of nezirus is not to escape from the mundane world, but to learn how to live in it properly.
Hopefully your Shavuos was celebrated with intense immersion in talmud torah and kabbalas haTorah, and so it’s only natural to feel a bit letdown on isru chag. Rabeinu Bachyei’s perspective reminds us that islands of spirituality are not meant as an oasis of temporary escape from the day to day, a momentary high from which we must fall, but are meant to give us the spiritual vitamins we need so our day to day becomes infused with that spiritual intensity and energy as well.