Charles Ellicott Commentary Deuteronomy 17:20

"that his heart be not lifted up above his brethren, and that he turn not aside from the commandment, to the right hand, or to the left: to the end that he may prolong his days in his kingdom, he and his children, in the midst of Israel." — Deuteronomy 17:20 (ASV)

To the end that he may prolong his days in his kingdom, he, and his children. — Shows that the kingdom in Israel would be hereditary only so far as Jehovah willed it to be so. Again we may say that the striking fact that no dynasty except that of David ever continued for more than five generations, and only two dynasties for more than two generations, while David’s dynasty was perpetual by promise, could hardly have escaped notice, if known to the writer of this book.