It is a matter of the heart, my brothers. There are many offices a man might fulfill as a king - father of a household, manager of a department, pastor of a church, coach of a team, prime minister of a nation - but the heart required is the same. "The king's heart is in the hand of the Lord; he directs it like a watercourse wherever he pleases" (Prov 21:1). The passage is often used to explain the sovereignty of God, in that he can do with a man whatever he well pleases. Certainly, God is that sovereign. But I don't think that's the spirit of this passage. God rarely forces a man to do something against his will, because he would far and above prefer that he didn't have to, that the man wills to do the will of God. "Choose for yourselves this day whom you will serve" (Joshua 24:15). What God is after is a man so yielded to him, so completely surrendered, that his heart is easily moved by the Spirit of God to the purposes of God.
That kind of heart makes for a good king.
Watch how Moses leads Israel out of bondage, and guides them to the Promised Land. Notice how every chapter telling the story of the Exodus begins, from chapter six to chapter fourteen: "Then the Lord said to Moses…" and the rest of the chapter is Moses doing what God told him to do. Is this how the men you know run their corporations, their churches, their families? I'm stunned by how little daily guidance Christian men seek from God. They have a good idea, and they just go do it. Not the great kings. Look at David. "In the course of time, David inquired of the Lord. 'Shall I go up to one of the towns of Judah?' he asked. The Lord said, 'Go up.' David asked, 'Where shall I go?' 'To Hebron,' the Lord answered. So David went up there…" (2 Samuel 2:1-2). In his heart, and in his daily practice, David is a man yielded to God. He is called, may I remind you, a man after God's own heart.
This is the way Jesus lived. "For I did not speak of my own accord, but the Father who sent me commanded me what to say and how to say it. I know that his command leads to eternal life. So whatever I say is just what the Father has told me to say" (John 12:49-50). Jesus could have asserted his own will; he certainly had the power to do so, and the talent, and we might add he also could be trusted to do so. But no - he was yielded to the Father, in all things. Regardless of age, position, or natural abilities, a man is only ready to become a King when his heart is in the right place. Meaning, yielded to God in all things.