A kohen is subject to the temptation of gaavah, because of his entitlements (the twenty four Matnos Kehuna) and his kedusha (which enables him to do the avodah and requires him to be tahor). Also, Kohanim are aware of everyone’s sins, because whoever brings a korban chatas has to be misvadeh; furthermore, when someone brings a chatas, he has to clearly explain to the Beis Din of the Kohanim why he is bringing it, so they can be sure that the Korban Chatas is indeed required and that it is not chulin ba'azara. So he might say lashon hora. This is a dangerous position to be in: you are born with superior kedusha, Klal Yisrael has to sweat to wrest a living from the earth while you sit at home and get your food-- grain, fruit and meat-- delivered tied with a bow, and you are privy to all their embarrasing failures and sins. It would not be surprising if Kohanim viewed the rest of Klal Yisrael as if they were a bunch of donkeys. This natural tendency to ga'avah and lashon hara can bring Tzara'as.
So the Torah says that the kohanim must personally look at nega’im. They need to see what the result of gaavah and lashon hara are. This constant visual reinforcement will help them control their yetzer hora. Very few oncologists smoke, and many dermatologists obssesively avoid exposure to sunlight, because day after day they see the deadly results of irresponsible and self destructive behavior; so, too, Kohanim are obligated to closely examine the nega'im of Tzara'as, and this will remind them to eschew the traits that bring Tzara'as - Ga'avah and Lashon Hara.
And this explains why the Avodas HaMilu'im recalls Taharas Metzora. The foundation ritual of Kehuna mirrors the taharas metzora process, so that every kohen will read this parsha and remember that the superior status he was granted brought with it a concommitant danger, and that every day he must be on guard against the temptations of ga'avah and lashon hara. Indeed, this concept is found in the Bracha the Kohanim give Klal Yisrael: Yevarechacha Hashem Veyishmerecha: every blessing brings along a heightened risk and the need for shemira. Kohanim, too, are blessed with many things, and these blessings create the need for greater shemira.
(Dr. Schreiber's words:
"...the similarity between the avoda of the taharas hametzora and the miluim of the kohanim which requires blood placed on the the bohanos of each of them. The kohen will hopefully carry the initial impression with him through his years of avodah and refrain from the failings that lead to one becoming a metzora.")
Please see the label list for many earlier Divrei Torah on Parshios Tazria and Metzora, starring the Ben Ish Chai, the Chidushei Harim, and others.