Why did David mention that Hashem chose him over her father's house?
Radak (20, citing Midrash Shmuel 25:6): She said, my father's kingship was nicer than yours. They would not let their hands or feet be revealed! David answered, Hashem chose me over your father's house. They abandoned Hashem's honor and engaged in their own honor. I abandon my honor and engage in my Maker's honor.
Malbim: You said that only officers and nobles should be close to the king. That is for a king that the nation chose. If they do not honor him, he is like a commoner. Hashem chose me. If the people chose, they would pick a descendant of your father, whom they anointed!
Why is the accent in "v'Sichakti" on the last syllable? Now, the Vov switches this to future tense!
Radak: More than what I did today, I will belittle myself in front of Him - "u'Nekalosi Od mi'Zos" (verse 22)!
Why did he say "Lifnei Hashem" twice?
Radak: Since Hashem honored me and chose me over your father's house, I lowered myself in front of Him, for His honor.
Malbim: A head general does not honor himself in front of the king, nor a slave in front of his master. How can a mortal king be honored in front of the King of kings, Hashem?! Therefore I was jovial in front of Him.