महाभारत - दूतः

कुलीनः शीलसंपन्नो वाग्मी दक्षः प्रियंवदः |
यथोक्तवादी स्मृतिमान् दूतः स्यात्सप्तभिर्गुणैः ||

- महाभारत

A messenger (mediator) needs to have the following seven qualities - be born in a virtuous family (and thus have like qualities), have righteous character, be a good orator, efficient, soft spoken, straight forward, have good memory. All these were found only in Krishna and hence was sent in to mediate between the Pandavas and the Kauravas.

- Mahabharatha