Rashi

I will tell of the decree Said David, “This is an established decree, and [one] that I have received to tell this and to make known.” The Lord said to me through Nathan, Gad, and Samuel. You are My son The head over Israel, who are called “My firstborn son.” And they will endure through you, as is stated concerning Abner (II Sam. 3:18): “for God said, etc., ‘By the hand of My bondsman David shall I deliver...Israel.’” And for their sake, you are before Me as a son because they are all dependent upon you. this day have I for I have enthroned you over them. begotten you to be called My son and to be beloved to Me as a son for their sake, as it is stated (I Chronicles 14:2): ‘And David knew that the Lord had established him as king over Israel, and that his kingdom was highly exalted for the sake of His people Israel.’ And we find that the kings of Israel who were beloved before Him were called sons, as it is stated (II Sam. 7:14) concerning Solomon: “I will be to him a father, and he shall be to Me a son.” We find further concerning David (Ps. 89:27) “He shall call Me, ‘You are my Father, my God, and the Rock of my salvation.’”