Because only if someone is impartial in his outlook will he be moved to treat all with impartiality.
فَإِنْ لَمْ نَكُنْ مُقْتَنِعِينَ أَنَّ عَلَيْنَا مُعَامَلَةَ ٱلْجَمِيعِ بِإِنْصَافٍ ،‏ فَلَنْ نَنْدَفِعَ إِلَى فِعْلِ ذٰلِكَ .‏