Judgement of the Heart

The Gemorah is Shabbos (139a) says that as reward for being happy for the appointment of Moshe and not bearing any feelings of jealousy, Aharon merited to wear the choshen mishpat.  Feeling happy about the appointment of Moshe, וראה ושמח בלבו is a tremendous act of carrying for someone else and indeed may merit a reward.  However, why is the reward the choshen mishpat, which  represents strict judgement?  It seems completely opposite of middah k'neged middah?  The explanation is that it is only one who has compassion that can be a good judge.  The judge that says יקוב הדין את ההר with no regards to the consequences can't be a proper judge.  Such an attitude can  lead to more harm than good.  It is the one that feels for their brother that is the proper judge.