In this past Sedra, Rashi mentions by the Rosh Chodesh korbon on the words "Chatas l'shem", that - kaviyochel - Hashem is asking 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 our history shows that it is full of failures... from Adam in Gan Eden, to Noach coming out of the teiva and getting drunk, to the shevatim with Yosef to the chet eigel and meraglim, to Shlomo building the Beis Hamikdash and oversleeping, to Chezkiyahu not singing shira, etc...
It seems that we can't ever do it right!
But the depth here is 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 [quoted above]... that the brightest light is the one that shines out of 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'HaShem, it is - kav'yochel - Hashem's fault, so to speak. He made it be this way! He wanted us 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 this Korbon as a monthly reminder so that we not get discouraged but keep climbing higher (and starting all over again if we slipped).
Hashem is telling us, "don't worry, everything will come to a tikkun, even mistakes that we don't know about... Just like the moon"... Don't worry about the morbid state of the world (or ourselves), Hashem says, "I did it, 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 a "once and for all - be over with it" type of 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 Korbon"; to remind us that it's His doing. He wanted it to be this way.