In Parshas Bamidbar we read of how God chose the tribe of Levi to be His, to a degree above and beyond the rest of the Jewish people. Originally, the Temple service was to have been performed by the first-born sons of the entire nation. When the people sinned with the golden calf, they lost this privilege and it was given to the tribe of Levi. The tribe of Levi was chosen because, at the time of the sin of the golden calf, they answered Moses' cry of "Whoever is for God, to me!" and took up their swords to punish the worshippers of the calf.
In his commentary on Bamidbar, Rav Avigdor Miller notes a surprising irony in this. At the end of his life, Jacob admonished his sons Shimon and Levi for the violent manner in which they avenged the honor of their sister, Dinah. Yet now the descendants of Levi were being rewarded for engaging in a violent battle against their fellow Jews!
Rav Miller explains that this teaches us an important lesson:
...we learn that no natural emotion or character-trait is intrinsically evil: "God made the Man right" (Koheles 7:29), but good or evil depends on the manner in which these emotions and character-traits are exercised. Anger and even cruelty, jealousy and ambition, indulgence and temperance, indolence and alacrity: each has its proper place, and when employed in Hashem's service all of these motivations gain recompense in this life and everlasting reward in the Afterlife. The anger which endangered Jacob's family was cursed and was punished by landlessness, but when the anger was utilized to combat idolatry it was rewarded by an eternal covenant: "The Levites shall be Mine."
This is an important lesson as we come into Shavuos, the festival of Matan Torah (the Giving of the Torah). One of the most basic messages of the Torah is that every aspect of human life has the potential for holiness. Judaism teaches us not to reject our natural drives and desires, but to channel them into the service of God. This is one of the basic symbolic messages of circumcision, which the Jewish people had to undergo before they could receive the Torah, and which every male convert must undergo to enter into the Jewish covenant with God. As Rav Samson Raphael Hirsch writes (Collected Writings III, pp. 78-79 - emphasis added):
All the physical aspects of of our earthly existence, with all its impulses and forces, its riches and pleasures, must be brought under the firm control of the holy will of God. This sign [of circumcision] poses, as the first and indispensable condition for our covenant with God that we must circumcise the ערלה [uncontrolled nature (lit. "foreskin)] of the physical aspect of our body. It is not the consecration of the spirit but the consecration of the body that marks the entry into the covenant of Abraham. This covenant categorically rejects the erroneous concepts of both extremes. It does not condone a mortification of the flesh one earth for the purpose of gaining life in the world to come. But it also rejects the worship of physical appetites and the cult of "beautiful" sensualism.