Before this section, in verse 10 it speaks of God's works giving thanks to him, and then it jumps straight into his Kingship. Why is that? I think it is because we are to be grateful that we have a king who is good, and awesome, all powerful, gracious, and merciful who is slow to anger and abounding in steadfast love who will rule over us forever!
What better king could we ask for? We should be thankful that this perfect king will never abdicate the throne!
In fact, and interesting side note; when Nebuchadnezzar finally comes to his senses and acknowledges the greatness of God and humbles himself by recognizing that God is King over him, he quotes verse 13 of this Psalm.