The Torah describes the symptoms of a malady which it calls צרעת, Tzara’at. Modern medicine cannot accurately identify this sickness. According to the Torah the Kohen has to examine the signs and determine if it is truly this affliction. Since the Kohen is not a doctor our sages recognize this is not a normal infection but rather an outward manifestation of a spiritual deficiency.
The Rabbis of the Talmud state that a Kohen is eligible to diagnose everyone’s symptoms except his own. This is a strange restriction since even a doctor may recognize his own symptoms. The Rabbis’ meaning, however, has a broader implication. They recognize that people can readily see faults in others but they cannot see their own faults.
This is why they maintain that the Kohen can diagnose the Tzara’at of others but not his own. Since this ailment is not a usual type of sickness but an indication of a spiritual deficiency, they are not able to judge their own status.