Or HaChaim on Deuteronomy 17:20/אור החיים על דברים י״ז:כ׳

1 לבלתי רום לבבו, "so that his heart does not become arrogant, etc." This may be understood as analogous to Avot 6,1 ומלבשתו ענוה ויראה, "and it clothes him in meekness and the fear of G'd." [The subject is a person who studies Torah with a pure purpose. Ed.] The Torah means that studying Torah as the king is bidden to do daily will ensure that he does not become haughty. Concerning the king developing fear of the Lord, the Torah had already written in verse 19: "in order that he will learn to fear the Lord his G'd."