‘Be kind’ has particular relevance to mediation. Science shows that you are much more likely to change another’s mind by arguing gently and kindly rather than belligerently. Furthermore, third-party observers always attribute high levels of competence to a person who conducts themselves with more civility.
As Lady Mary Wortley Montagu put it in the 18th century: ‘Civility costs nothing and buys everything.”
This Guardian article describes other ways to be more successful influencing others and includes links to supportive research.