On his behalf

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When a person has the physical symptoms of the spiritual disease Tzara'as... he must call out: ‘[I am] Impure! [I am] Impure!' (VaYikra 13:45). The Gemara inquires why the Metzora is required to call out at all and why use the double expression: ‘[I am] Impure! [I am] Impure!'? (Mo'ed Katan 5a). The Gemara answers that one mention of ‘Impure' is to notify people to stay away so they do not become contaminated. The other mention of ‘Impure' is to inform the public to daven to Hashem for his recovery.

Why does the Metzora need others to daven on his behalf? Why can't he just daven himself? The classic cause for contracting this Tzara'as disease is the habitual speaking of Lashon Hara (Rambam, Tumas Tzara'as 16:10). Lashon Hara essentially refers to non-constructive statements that are either derogatory or potentially harmful to a third party. The Zohar explains that Hashem will not listen to the prayers of habitual speakers of Lashon Hara. That is why the Metzora needs others to daven for him. Deliberately looking for the faults, rather than the merits, of others and talking about them to one's friends brings with it the horrific consequence that such people will not be listened to by Hashem, however strongly they daven and cry out and however many other good qualities that they may have.
http://www.revach.net/parshas-hashavua/quick-vort/Parshas-Tazria-The-Tefilos-of-A-Metzora/2074