Whom the Lord thy God shall choose Approve of, or appoint. So it was in Saul and David. God reserved to himself the nomination both of the family and of the person. Thy brethren Of the same nation and religion; because such a person was most likely to maintain true religion, and to rule with righteousness, gentleness, and kindness to his subjects; and that he might be a fit type of Christ, their supreme king, who was to be one of their brethren.
Related Commentaries of Deuteronomy 17:15
Deuteronomy 17:15
15 Thou shalt in any wise set him king over thee, whom the LORD thy God shall choose: one from among thy brethren shalt thou set king over thee: thou mayest not set a stranger over thee, which is not thy brother.