If a person develops ... tzara'at on the skin of his flesh, he must be brought to Aaron the high priest, or to one of his sons, the regular priests, or their successors, for only they are authorized to pronounce someone or something defiled or undefiled on account of tzara'at.
For only they are authorized to pronounce someone or something defiled
It is the priest's pronouncement--rather than the appearance of qualifying symptoms per se--that render the person ritually defiled. This fact puts considerable onus on the priest, especially in light of the consequences of his pronouncement: the ritually defiled individual must be banished from society, even from the company of other ritually defiled people. This total ostracism is not required in the case of any other type of defilement.
The Torah therefore specifically requires a priest to make this pronouncement. The priests are all descendants (and therefore spiritual heirs) of Aaron, whom both embodied the ideal of brotherly love and promoted brotherly love among the people. G-d therefore refers to the priests as His men of loving-kindness and grants them the privilege of blessing His people daily. Moreover, the blessing tradition requires them to recite before pronouncing this blessing is "Blessed are You, G-d, our G-d, He who has sanctified us with the holiness of Aaron and commanded us to bless His people, Israel, with love."
Imbued with this love for their compatriots, the priests--while remaining objectively true to the Torah's directives for determining if a given outbreak of symptoms renders the sufferer defiled or not--will make all efforts to ensure that the law indeed requires them to pronounce the sufferer defiled before doing so. Furthermore, their inherent love for their fellows will compel them to do whatever it takes to declare them undefiled at the earliest possible opportunity.
The lesson for us here is that when we encounter someone whose behavior makes us judge him to be unfit to be included with us or befriended by us, we should not rush to declare him so. Rather, we should first examine ourselves in order to determine how well we exemplify the ideals of brotherly love. If we are in any way lacking in this regard--if we are not a "priest, a descendant of Aaron"--we have no right to pass such judgment, for it could well be that our perception is skewed by our unrefined sentiments rather than grounded solidly in the objective laws of the Torah.
Moreover, anyone who is less than a "priest"--an embodiment of brotherly love--is not qualified to ostracize another Jew, and if he presumes to do so, his pronouncement is no less than an outright lie, for as stated, it is only the pronouncement of the priest that renders the individual defiled (and therefore subject to exclusion from society), not the symptoms themselves.
It thus follows that someone who utters such an unauthorized judgment has slandered his fellow, which, as we have seen, results in him being afflicted with tzara'at, rather than the person he sought to stigmatize.
Therefore, in order to purify himself of this defilement, the judgmental person should isolate himself from social contact until he trains himself to see only the positive in his compatriots.
By learning how to love our fellows "unwarrantedly"--i.e., positively, regardless of their objective behavior--we counteract the cause of our present exile, unwarranted hatred. Thereby, we hasten the advent of the final, messianic Redemption.