Rebuke

"He should be brought to Aharon the Kohen..." [Tazria 13:2]

The Kohanim (priests) were people of inherent kindness who blessed the Jewish people with love.  Therefore, when it came to declaring somebody with the severe condition of tzara'as, which required total isolation from the Jewish camp, it was imperative that this harsh judgment be done out of love, so the Torah required it to be done by a Kohen.

From this we can learn a powerful lesson: that if one feels that another person has acted disgracefully and one wishes to chastise him, one must first examine one's own motives to see if one's desire to rebuke another is truly being done out of love.

Source: Likutei Sichos Lubavitcher Rebbe