The lesson from this applies to one who encounters a Jew and sees in him something lowly and unbecoming, G-d forbid, seeing him as an outcast, not worthy to be among the encampment of the Jewish people, G-d forbid. In that situation, the Torah states that prior to rendering this ruling upon another Jew, even a great sage must first make a proper self-assessment of where is he holding in kindness and love of a fellow Jew. * Likkutei Sichos Vol. 27, pg. 88-91
Translated by Boruch Merkur
1. Concerning the impurity of “nega’im – lesions [of miraculous origin],” there are two general rules:
1) The examination of the nega (establishing whether or not the nega is impure) must be done by a sage, one who “was taught [the relevant laws] by his master and is adept in all nega’im and their classifications, regarding all nega’im afflicting people, as well as nega’im that afflict garments and houses” (Rambam Laws of the Impurity of Tzaraas 9:2, end). The sage does not have to be a Kohen. Rather, “all [sages] are acceptable to examine nega’im.”
2) “[The actual status of] impurity and purity is dependent upon a Kohen” (ibid, beg.). That is, even after the sage has advised that he has detected an impure nega, the one afflicted is not rendered impure until the Kohen states “(you are) impure.” (Similarly with regard to the purification of the one afflicted – even after the person (or garment) is healed of the nega, he remains “in a state of impurity until the Kohen tells him ‘you are pure.’” (Ibid 9:3, end))
The Kohen’s articulation of the diagnosis is a necessary condition (in establishing the impurity or purity of the one afflicted). In fact, even a “Kohen who does not know how to identify [nega’im] (and “even if the Kohen is a minor or is mentally deficient”) may “rely upon the words of the sage” [and pronounce the person pure or impure on the sage’s advice]. “The sage examines [the nega] and tells him ‘say that it is impure,’ and the Kohen says ‘impure’; [or the sage tells him] ‘say that it is pure,’ etc.”(ibid 9:2)
The following two points, however, require explanation:
1) Since the Kohen “relies upon the words of the sage,” why is the Kohen’s articulation necessary?
2) What is unique about tzaraas that the Torah states specifically about this form of impurity the rule that “impurity and purity is dependent upon [the articulation of] a Kohen”?
True, this law is classified as “g’zeiras ha’kasuv – a Scriptural decree [that is not subject to logic],” however, it is well known that Rambam writes that even regarding “chukei Torah” which “are decrees,” it is proper for one to contemplate them and provide a rationale for them to whatever extent possible.” This is especially the case with regard to finding underlying reasons that lend themselves to “correcting one’s character” (as Rambam writes, “the majority of the laws of the Torah are strictly advice…to correct one’s character and straighten out one’s behavior”), or as the verse states, “G-d has commanded…all these laws (chukim) to fear G-d,” laws that teach a lesson on the fear of Heaven.
2. Clearly, the main innovation of the Torah with regard to the rule that “impurity and purity is dependent upon a Kohen” is expressed (not regarding the purification process of the metzora, the one afflicted with tzaraas, but primarily) in his rendering another Jew impure:
The purification of the metzora comes about only after his impurity had previously been established. Thus, it is logical to say that the Kohen’s capacity to purify the metzora through speech is a result of having initially diagnosed the impurity of the lesion. (That is, since the impurity is only established through the articulation of the Kohen, therefore, also the purification (from the tuma) is established specifically through the articulation of the Kohen.)
The following, however, remains elusive: It goes well if the main point of the Torah regarding diagnosing tzaraas would be in connection to the process of purification, for then we would understand why it is dependent upon a Kohen, insofar as the Kahuna, the Priesthood, is connected with the state of purity (as reflected by the rule that a Kohen may not defile himself). Indeed, it is for this reason that Kohanim are the one who impart purity to other Jews.
However, why is specifically a Kohen needed in order for a Jew to be deemed impure?!
3. One of the explanations of the matter:
There is a unique stringency applicable to the impurity of tzaraas. Namely, that “he shall dwell in solitude; outside of the encampment shall be his dwelling” (Tazria 13:46) – he is cast out of the three encampments (Rashi on the verse). And there, outside the camp, he must dwell in “solitude,” so that he does not enjoy the company of other impure individuals (Rashi). (Indeed, he is even not to have contact with other metzora’im ––Likkutei Sichos Vol. 22, pg. 74 and in footnote 49.)
In this sense, the spiritual significance of the impurity of tzaraas is that it is such a severe defilement that the afflicted individual is wholly evicted from the encampment of sanctity [i.e., from among the Jewish people]. It is as though he were an outcast from the holy nation, may G-d have mercy on us.
In response to this, the Torah says: Who is it that can rule about another Jew that he is, in effect, cast out, G-d forbid, from the encampment of holiness? Only a Kohen.
A Kohen is an expression of the concept “to bless the Jewish nation with love.” A Kohen is “a man of kindness” (Bracha 33:8), one who blesses Jews – and with love. (Indeed, it is an essential condition that pertains to Birkas Kohanim, the Priestly Blessing, that it must be done with love. (See Shulchan Aruch HaRav Orach Chayim 128:19). In fact, it is actually a danger for a Kohen lacking in love [of his fellow] to bestow the [Priestly] blessing.) Thus, the Torah relies specifically on the fact that the Kohen is “a man of kindness” in order to render the judgment upon a Jew that “outside of the encampment shall be his dwelling.”
(The above sheds light on the further Scriptural references to the topic: “The Kohanim…should approach, for G-d, your L-rd, has chosen them to serve Him and to bless in the name of G-d, and by their word shall every controversy and every nega be [judged]” (Shoftim 21:5). Specifically on account of the fact that “G-d, your L-rd, has chosen them…to bless in the name of G-d, (therefore) by their word shall every controversy and every nega be [judged].”)
Of course, it is imperative that the Kohen’s ruling be founded only upon the Torah, for which reason there must first be the examination of the nega by a sage, who applies the wisdom of the Torah and who is “adept in all nega’im, etc.” (Indeed, it must be a sage who has received the knowledge as a tradition from his master, his teacher.)
However, when it comes to actually determining the practical application of the ruling, it must be done through a Kohen, “a man of kindness,” for only he is sensitive enough to feel the full force of rendering such a negative halachic ruling for another Jew. The Torah is confident that the Kohen will not withhold any efforts in the process of consulting with the sage about the [correct] ruling of the Torah, arousing his sensitivity to the concept of “and they shall judge the congregation – and save the congregation” (P’sachim 12a, beg.).
(The latter is reminiscent of the concept “there is no one as wise as the one who has been put to the test.” When something is studied, knowing in advance that an instruction and practical ruling will be determined from the analysis of the topic, one applies himself to delve deeper into it until he uncovers the truth of the matter.)
And if after arduous deliberation, analysis, and scrutiny, the Kohen, the man of kindness, resolves and proclaims “tamei – impure,” it is then that we are certain that this is the true ruling of the Torah. From another perspective, we are confident that [after the one afflicted has been diagnosed as being impure] the Kohen will fully apply himself to ensuring the subsequent purification of the metzora.
4. The lesson from this applies to one who encounters a Jew and sees in him something lowly and unbecoming, G-d forbid, seeing him as an outcast, not worthy to be among the encampment of the Jewish people, G-d forbid. In that situation, the Torah states that prior to rendering this ruling upon another Jew, even a great sage, proficient in the entire Torah, and according to his estimation of the wisdom of the Torah, this person is subject to the ruling of “outside of the encampment shall be his dwelling” – this sage must first make a proper self-assessment of where is he holding in the trait of kindness and in love of a fellow Jew. Indeed, if he is lacking, G-d forbid, in true love of his fellow, he has no right to issue such a ruling on another Jew! In fact, his ruling puts into question whether it was derived from pure Torah considerations or perhaps it emerged as a result of having unrefined character traits, etc.!
The Torah states that the nega itself does not establish impurity. Rather, the ruling and articulation of the Kohen (“man of kindness”) of the words, “you are impure,” establishes the impurity of the metzora. Thus, one who is not a Kohen, a man of kindness, and rules about another Jew that “outside of the encampment shall be his dwelling,” is lying. And therefore, it is he who is the “metzora” [insofar as he is] slandering [another Jew] (as our Sages say on the verse, “This shall be the law of the metzora” – “this shall be the law of the one who slanders another Jew”). Not only has he spoken negatively about another Jew (for lashon ha’ra, talking negatively about another, applies “even when one says the truth” – Rambam’s Laws of Proper Character 7:2), regarding whom the punishment is nega’im (tzaraas) (Erchin 15b, 16a), he is [doing something even worse] slandering, lying …about his fellow” (ibid).
What is the correction for this fault? “He shall dwell in solitude; outside of the encampment shall be his dwelling.” Until this person is inclined to look at another Jew in a positive light, he must separate himself from other Jews, thereby not causing grief with his wicked talk and slander!
Only when this person is healed of his “nega,” only then may he return and be healthy.
But through conducting oneself with true love of one’s fellow, in a manner of unwarranted love – that instantly nullifies the cause (baseless hatred –Yoma 9b) of exile, which is referred to as “ha’tzarua,” bringing about, as a direct result, the true and complete Redemption through Moshiach Tzidkeinu, speedily, literally in our days.
(From the address of Shabbos 
Parshas Toldos 5745)