Do you get goose pimples when you are scared, or a nervous stomach when you are excited? Isn't that strange? After all, it's your mind and heart that feel excitement or fear. Why do your hands and stomach react in this way?
HaShem created our bodies to be sensitive and to react to the things which we think and feel.
The Torah tells us that there was a time when people were so sensitive, that they could actually see a change on their skin because of something they said. When a person would speak bad things (lashon horah) about other people over and over again, blotches of discolored skin, tzoraas, would appear on his body. The Torah calls this person a metzora. Metzora is short for motzi shem ra - saying bad things about someone else.
Tzoraas is a type of tumah, impurity, from which the metzora had to purify himself. If a person discovered it on his body, he had to leave the entire camp of the Jewish people and follow the Torah's instructions for purifying himself. He separated one Jew from another. Therefore, the Torah tells him that he must stay away from the rest of the people until he becomes purified.
If the metzora has to stay alone, away from everyone else, who is going to help him become pure? A kohen. Kohanim are kind and loving ahavas Yisrael Jews. Listen carefully to Birkas Kohanim - the Priestly Blessing - in shul. The kohanim say: "Baruch Atah HaShem...who has commanded us to bless His nation, Israel, with love."
The metzora did not show Ahavas Yisrael when he spoke badly of others. He needs to be shown the right way to act - and kohanim, with their strong love for their fellow Jews, can do this job best. Because he cares for another Jew, even a metzora, the kohen goes out from the Beis HaMikdash, the holiest place in the world. He walks to the outskirts of the city, to the place where the metzora is staying, to help him become pure and come back to his community with more ahavas Yisrael than he had before.
(Adapted from Likkutei Sichos, Vol. 27, p. 88ff)