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.
So that his pride doesn't grow more important than others and he doesn't disobey rules by going left or right, this will help him stay in power longer, for himself and his kids, all within the nation of Israel.