Teshuvah
The Gemara (Shabbos 89.) states that after matan Torah, the Satan went to Hashem and asked, "Where is Your Torah"?
Could it be that the Satan didn't know of matan Torah? When Hashem gave the Torah, His Shechinah came down onto Har Sinai, and Hashem opened all seven heavens, and the entire world knew about it. The malachim gave presents to the Jewish nation, and even the Satan gave a gift to the Jewish nation. So, why did he ask, "Where is Your Torah?"
My grandfather, Rebbe Moshe Mordechai of Lelov zt'l answers that the Satan’s question is directed to the Jewish nation, and he asks the question every year to every Yid. He asks, "You received the Torah a year ago. Where is that Torah that you received? It doesn't seem that you are studying it or keeping its mitzvos." With these words, the Satan tries to discourage the Jewish nation. But we must know that we can always do teshuvah. We can repent and improve. This year, we can truly receive the Torah.
Most foods become ruined when they spoil, but milk becomes an even better product when it spoils—it becomes cheese. On Shavuos, we eat cheese and milk products to remind ourselves that even if one sinned and spoiled his ways, he can always do teshuvah and receive the Torah. There is still hope for him.
Chazal (Niddah 9:) tell us חלב ונעשה נעכר, "blood becomes spoiled, and turns into milk." The origin of milk is blood, which transforms itself and becomes milk. It is forbidden to eat blood, so why is it permissible to drink milk? It must be that when it turns into milk, the blood disappears.
The Brezhaner Rav zt'l says that this symbolizes teshuvah. When we improve our ways, there remains no sign of our past sins.
The Yerushalmi (Rosh Hashanah 4:5) states, "By all korbanos it states לחטאת עיזים שעיר, a goat for a sin-offering.' But by the korbanos of Shavuos it states עליכם לכפר עיזים שעיר, 'a goat for atonement.' חטא isn't mentioned because, on Shavuos, no one has sins. Hakadosh Baruch Hu tells Bnei Yisrael, 'Since you accepted the yoke of Torah, it’s as though you never sinned your entire lives.'"
The Torah writes (Shemos 19:4) על אתכם ואשא נשרים כנפי, "I carried you on eagles' wings." The Yismach Yisrael (Yisro 6) explains that an eagle sheds its feathers and grows new ones every year, as it states (Tehillim 103:5) כנשר תתחדש נעוריכי, "Become new, your youth, like an eagle." Rashi writes, "Like an eagle that replaces its feathers and hair yearly." Similarly, the Torah tells us that Hashem carried us on eagle’s wings to remind us that we should constantly change and not remain stuck in our old ways.
