In this past sedra Rashi mentions by the Rosh Chodesh korbon: Chatas l'shem, that kaviyochel Hashem asks us to bring a korbon for him that he made the moon smaller.
There obviously must be a lesson here... Hashem does not need our kapara!
The making of the moon smaller represents the downtrodden status of klal yisroel and holiness in the world (the moon = klal yisroel).
A glance at history shows that it is full of failures... from Adam in Gan Eden, to Noach coming out of the teiva, to the shevatim with Yosef to the chet eigel and meraglim to Shloma building the B"M and oversleeping to Chezkiyahu not singing shira etc...
It seems that we can't ever do it right!
But the amkus here is (heard this idea from R' A. Miller and lahvdil B'CH"L"CH from R' Wolfson and also R' Akiva Tatz) that it was all preordained to be this way... Hashem wanted all these falls because he treasures the light that (will ultimately) comes from darkness. It is much sweeter like the Zohar says... the brightest light is the one that shines in the pitch dark.
So on Rosh Chodesh when the moon starts coming back from one of the yeridos, we bring a chatas korbon, meaning we start the process of coming close to Hashem again, and whose fault is it? Chatas l'Shem, it is kavyochel Hashem's fault so to speak, he made it be this way, to take the long route rather than the easy way up. But the lesson for us is that this is the way it is meant to be, and we have the monthly reminder so that we not get discouraged, but keep climbing higher (and doing it yet again if we slip).
R' Wolfson often likes to show how the pashut p'shat in a Rashi jives with the drush. The pashut pshat here is that the korbon is going on a cheit that only Hashem knows about... in other words, an error that the person never discovered, and never therefore could bring a korbon for.
Well, this is essentially the same idea... Hashem is telling us that don't worry, everything will come to a tikun, even mistakes that we don't know about... same idea as the moon... don't worry about the morbid state of the world (or ourselves), Hashem says I did it, and so you can be sure that I'll help you out of it.
We so often feel sick with ourselves... why do we have to have all this garbage inside? The moon is teaching us that our mission in this world is constant renewal. Not just: "once and for all be over with it" avodah, but constant ups (and yes downs). We are living in sync with history and with the ultimate purpose of the world. And every up is infinitely brighter because of it.
Yes, its hard and frustrating. So Hashem says bring a karbon, to remind us that its His doing. He wants it to be this way.