Deuteronomy 17:20

20 neither his heart be raised into pride on his brethren, neither bow he into the right side, either left side, that he reign long time, he and his sons on Israel. (and then his heart shall not be raised up in pride above his brothers, or his kinsmen, nor shall he turn from these commandments to the right, or to the left, and then he and his sons shall reign a long time over Israel.)

Deuteronomy 17:20 Meaning and Commentary

Deuteronomy 17:20

That his heart be not lifted up above his brethren
On account of his office, the dignity of it, considering that he was subject to the law of God, and accountable to the Lord for all his actions:

and that he turn not aside from the commandment, to the right hand or
to the left;
not in the least deviate from the law of God in the whole of his conduct, and particularly in the exercise of his kingly office:

to the end that he may prolong his days in his kingdom;
ruling well according to the laws of God being the way to rule long:

he and his children in the midst of Israel;
this shows, as Jarchi observes, that if his son was fit for the kingdom, he was to be preferred to any other man; for though it was elective, yet to be continued in the same family, provided they walked in the ways of the Lord, and observed his laws.

Deuteronomy 17:20 In-Context

18 Forsooth after that he hath set in the throne of his realm, he shall write to himself, that is, shall make to be written, the deuteronomy, that is, declaration, of this law in a book, and he shall take (the) exemplar of (the) priests of the kin of Levi; (And after that he hath sat on the throne of his kingdom, he shall have written for himself the deuteronomy, or the declaration, of this law in a book, and his copy shall be made from the original held by the levitical priests;)

19 and he shall have it with him, and he shall read it in all the days of his life, that he learn to dread his Lord God, and to keep his words and his ceremonies, that be commanded in the law; (and he shall have it with him, and he shall read it all the days of his life, so that he can learn to fear the Lord his God/so that he can learn to revere the Lord his God, and obey all his words and his statutes, that be commanded in the law;)

20 neither his heart be raised into pride on his brethren, neither bow he into the right side, either left side, that he reign long time, he and his sons on Israel. (and then his heart shall not be raised up in pride above his brothers, or his kinsmen, nor shall he turn from these commandments to the right, or to the left, and then he and his sons shall reign a long time over Israel.)