Chazal explain that the Torah put the parsha of nazir right next to the parsha of sotah to teach us that . someone who sees a sotah fall into disgrace should respond by taking a vow of nezirus. Everybody asks: it’s the person who doesn’t see the downfall and disgrace of the sotah and who needs to be extra careful and maybe take a vow of nezirus. The person who sees the sotah sees with his own eyes the effect too much wine can have and knows the danger!
The Akeidah gives a brilliant answer: it’s davka the person who sees the miraculous power of the sotah water and the whole process of her punishment and therefore thinks that he/she has learned the lesson and is immune from danger who needs the extra reminder.
“Machisi k’av pesha’echa v’k’anan chatosecha shuvu eilai ki g’altich.” (Yeshayahu 44:22). The simple pshat in the pasuk is that because Hashem has forgiven all our sins therefore we should return to him. The Shem m’Shmuel, however, reads the word “ki” not as meaning "because," but rather as meaning af al pi, "even though." Another example of the same use: Moshe Rabeinu asked Hashem to forgive Bnei Yisrael after the cheit ha'eigel, "Ki am k'shei oref hu..." -- not because they are a stubborn people, but despite/even though they are a stubborn people (see Ibn Ezra).
We've gone through a Yom Kippur and Hashem has forgiven all our sins. For some people, that means it's time to breathe a sigh of relief -- it's all over and I made it! I got by spiritual flu shot for the year and can now get back to business as usual. So the Navi tells us, "Shuvi eili ki g'altich" -- even though you spent the day properly and were forgiven, you need to focus on teshuvah. Davka because you had such a wonderful Yom Kippur, there is a danger of spiritual complacency setting in, of thinking you've done your part and that's all there is. Yom Kippur has to be a beginning, not an end.