Tazria-Metzora

He shall be brought to Aharon the kohen, or to one of his sons the kohanim.[13:2]

On this verse the Meshech Chochmah writes that we have no easy, apparent explanation for why Aharon is singled out to pass judgment on what looks like a medical symptom.

Furthermore, because the examination of the metzora takes place outside the precincts of the beis ha-mikdosh, it cannot be considered avodah in the usual sense.
This makes it one of two examples (along with the preparation of the parah adumah) of procedures that are not part of the avodah yet nonetheless require a kohen.

We can suggest the reason for the Torah’s insistence on a kohen by noting that nega’im were held to be terribly contagious.
The midrash. [Vayikra Rabba 16:3] speaks of precautions that great people took to distance themselves from those stricken by nega’im. One refused to come closer than a hundred amos; another spurned food coming from the same alleyway; yet another would not walk into an alleyway shared by a nega-victim.

Our parshah instructs the metzora to call out, “Tameh! Tameh!” – apparently as a warning for others to give him a wide berth.

Having established the danger in any contact with the metzora– candidate, we can understand the Torah’s insistence on the kohen as the examiner. How are we to obligate public servants to expose themselves to considerable risk in ministering to the metzora?

Our best candidates will be those who enjoy a special kind of Divine providence and protection. Kohanim are quite often treated as members of a subgroup who stand apart from other Jews. They have special roles, for which they ready themselves through special restrictions and responsibilities. Their special status allows for a Divine oversight that is more focused and attentive, as it were, to their needs.

They are the ones who can best afford to take the risk of contagion from the metzora.

Shabbat shalom