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.
God was going to give Israel a king. The concern was that kings begin to think too highly of themselves. They were given instructions that should help the king realize that he was still human, one of the people. God simply wanted the king to do right. God wanted to bless the king due to his obedience. So God instructed the people and the king in what to do in order not to get lifted up in pride.
They were only to let one of their own lead them. The king was not to make himself rich. He was not to take God’s people back to Egypt. Many times Egypt is a picture of the world in the Bible. The king was to lead God’s people to do right, to honor God, never to turn to the world. He was not to have a multiplicity of wives. Basically, he would not make alliances with other countries by marrying the daughters of foreign kings.
He was also to make himself a personal copy of the law, the books of Moses, the Pentateuch. He was to read in the book of the law every day. He was to learn to respect and reverence God. He was to live in obedience. He was to do what the Scriptures said to do. This would keep him from being lifted up in pride.
This is good advice for all of us as we seek to love and serve God. We do not need alliances with the world. We do not need to go to the world to get help. We need to be in the Bible every day. We need to obey God humbly.