1)
Why did David himself offer the Korbanos?
1.
Malbim (Shmuel II, 6:17): He was aroused to do so amidst Simchah of Mitzvah, to draw an influence of Hashem's desire.
2)
Why did David bless the people b'Shem Hashem?
1.
'Rashi': It is because Hashem's name is called on the Aron ? "Aron ha'Elokim Hashem Yoshev ha'Keruvim" (13:6).
2.
Malbim (Shmuel II, 6:18): Because he received such an influence from Hashem, he gave to the people spiritually (a Brachah) and physically (bread, meat and wine ? verse 3).
