John Gill Commentary 2 Samuel 7:21

"For thy word`s sake, and according to thine own heart, hast thou wrought all this greatness, to make thy servant know it." — 2 Samuel 7:21 (ASV)

For your word's sake
For the sake of the promise he had made to him by Samuel, that he should be king, and his kingdom should be established; or for the sake of the Messiah, that should spring from him; the Memra, as the Targum, the essential Word of God; and so the Septuagint version, "because of your servant", with which agrees the parallel text in (1 Chronicles 17:19);

and according to your own heart ;
of his own sovereign good will and pleasure, of his own grace, as the Arabic version, and not according to the merits and deserts of David:

have you done all these great things ;
in making him king of Israel, and settling the kingdom in his posterity to the times of the Messiah, who should spring from him:

to make your servant know [them] ;
as he now did by Nathan the prophet, what he and his should enjoy for time to come; so that it is not only a blessing to have favours designed, purposed, and promised, but to have the knowledge of them, to know the things that are freely given of God.