To what word of Hashem does he refer?
Rashi: You told Shmuel to make me king.
Why did he say "uch'Libcha"?
Rashi: It is Your desire; I am not worthy.
Radak: "Ba'avur" applies also to this. It is like a repetition of "Devarcha", to strengthen the matter.
Why did he say "Asisa
Malbim: You strengthened the matter via telling me through a Navi. A Navi is sent only to warn about future evil, so people will repent, or to warn and counsel what to do or not do, or similar matters. Hashem does not send a Navi to an individual to tell what will happen to him! He is sent only for a need (or to establish the Navi to be a true Navi through a Nevu'ah for good). Why did Hashem send a Navi to inform David? This shows that this is a matter for the perfection of the nation from the beginning to the end, like all the prophecies about the days of Mashi'ach and the world to come. They were not only for that generation, rather, to reveal His intent and conduct [of the world] from the beginning to the end.
