The first part of Proverbs 21:15 could (and probably does) have a dual meaning. The American Standard Version reads, “It is joy to the righteous to do justice.” The English Standard Version reads slightly different, “When justice is done, it is a joy to the righteous.” According to the ESV, when the righteous see justice administered (perhaps by government or some other authority), it brings them joy, for the righteous are very interested in that which is right. However, the emphasis of the ASV, the KJV and the NKJV makes the passage a little more personal, by pinpointing the joy experienced by the righteous when they themselves “do justice.”
It is easy (and sometimes “enjoyable”) to retaliate, to react angrily, to return evil for evil, to seek revenge when someone has done us wrong. But, for the righteous, God says there is actually “joy” when we treat someone else fairly and do right to them (according to God’s standard, rather than the world’s standard). How much joy have you had lately in doing right to someone, particularly when they have not done right to you? Find God’s joy in that.