Why does it say "Karati Bris li'Vchiri"?
Radak: He told David via Nasan ha'Navi "v'Chonanti Es Kis'o Ad Olam" (Divrei ha'Yamim I, 17:12). "Bechiri" is David - "va'Evchar b'David Lihyos Al Ami Yisrael" (Divrei ha'Yamim II, 6:6).
Malbim: Even though we perceive Hashem's Chesed in the Klal of the world, we received from Nevi'im that it is also for individuals. He chose David amidst Chesed, without considering deeds 1 , and fulfilled His word, amidst Midas ha'Emunah.
Surely His choice of David was not arbitrary! We find that Hashem would have chosen Eli'av, but rejected him, for later he would get angry (Pesachim 66b, based on Shmuel I, 16:7)! (PF)
Where did Hashem swear so to David?
Radak: Refer to Shmuel II, 3:9:1:1-3 and the notes there.
