Albert Barnes Commentary 2 Samuel 7:19

"And this was yet a small thing in thine eyes, O Lord Jehovah; but thou hast spoken also of thy servant`s house for a great while to come; and this [too] after the manner of men, O Lord Jehovah!" — 2 Samuel 7:19 (ASV)

Is this the manner of man - Compare 1 Chronicles 17:17. This passage can be understood as follows: it is the law (or prerogative) of a great man to establish dynasties that are to last far into the future. David expresses his astonishment that he, a man of such humble birth and so insignificant in his own eyes, should not only be raised to the throne but also be assured of the perpetual succession of his descendants, as if he were a man of high standing.