The most curious feature of the Yom Kippur davening occurs after Yom Kippur ends. In the Shmoneh Esreh of Maariv we recite, as we usually do, a blessing asking God to forgive us for our sins. This would seem to be a curious thing considering we just spent many of the previous 25 hours begging for forgiveness from the Ribono Shel Olam, a task in which we were hopefully successful. Unless one really tried hard to get in a few words of loshon horo between the end of Neilah and the beginning of Maariv, or one couldn't wait until havdalah to have that first bite of chocolate, how could we justify once again asking for forgiveness so soon?
The Rav, ztk"l, when discussing the reason we don't fast on Rosh HaShanah but, adraba, treat it like a Yom Tov even though we are standing before God on that day and having our lives judged, notes that the very fact we have a Rosh HaShanah in the first place points to the special privilege the Jewish people have in all of Creation. Alone amongst all the forms of life God created, only humans are aware of God and His rule over the universe. Alone amongst all the nations of the world, only the Jewish people relate to him as a King on Rosh HaShanah and actively submit to that judgement. Awareness of this privilege is the reason for celebration on Rosh HaShanah.
But awareness is only half of the process. The other half is, of course, teshuvah. We spend the next seven days until Yom Kippur engaged in the recitation of selichos and on Yom Kippur completely submerge ourselves into thoughts and actions that disengage us from the physical world so we can maxmize our return to God.
But do we ever actually achieve it? At the end of Yom Kippur we stand like angels before God, dressed in white, fasting and barefoot but an instant later the magic ends. How many people drag Maariv out so as to keep themselves at that level? How many regret that first bite of food after davening as opposed to rushing the table, elbowing the little old people out of the way, so as to get that first bite of kugel? And if this is so, if we are willing to toss away 25 hours of effort to come closer to God the first chance we get, did we achieve real teshuvah at all?
Rav Avraham Kook, ztk"l, writing in Orot HaTeshuvah, brings a different perspective to this. For him, teshuvah isn't so much a destination as the journey. A person does not so much achieve teshuvah as reach towards it on a constant basis through regret for sins and efforts to improve one's thoughts and actions. As a result, the end of Yom Kippur is not the end of our journey which only ends with real teshuvah, the returning of the soul to God in Heaven. Yom Kippur may advance us a great deal each year of our lives on the journey but the journey continues the instant Yom Kippur ends which means Yom Kippur itself was part of the process.
As the Rav also notes, Yom Kippur is a day for forgiveness from sin but there is no guarantee we cannot sin on that day. Did our kavannah waver? Did we talk at an inappropriate time during davening? Did we speak loshon horo? And even if one might say that such deviations were unintentional and minor compared to the sins we were confessing for, if we are like angels on that day are even our minor sins that much more serious given our higher level?
If that is so, then the Slach Lanu Avinu we say in the Shmoneh Esreh immeidately after Yom Kippur now makes perfect sense. We are not done standing before the King of the Universe, as per the Rav, and our journey towards real teshuvah is continuing, as per Rav Kook. By again acknowledging our imperfection and need for constant forgiveness from our loving Avinu Shebashamayim, we recognize that Yom Kippur is part of the ongoing process of the improvement of our souls and that we are not eager to leave the King's presence and return to a mundane life.