What is the meaning of "Ahavta Tzedek"?
Malbim: You conduct kingship with straightness and Tzedek. This became a Kinyan in your Nefesh, to the point that you love Tzedek and hate evil. Therefore, it is impossible for you to veer from Tzedek and straightness.
Malbim (Derech ha'Muskal): Due to the straightness planted in you heart, you love Tzedek and hate its opposite. Therefore, Elokim anointed You with oil, i.e. Divine influence and Chachmah.
Why does it say "Meshachacha Elokim"?
Rashi: Every expression of grandeur, anointing with oil applies to it, like the protocol of [anointing] kings.
Why does it say "Shemen Sason"?
Radak: The entire world rejoiced when Elokim anointed you.
Malbim: Refer to 45:8:4:3.
Why does it say "me'Chaverecha"?
Radak #1: Hashem chose you to be king from among your colleagues.
Radak #2: Hashem chose you more than your colleagues. Chaverecha is plural, even though it is written without a Yud; there are many verses like this.
Malbim: Your lovers will anoint you with oil of joy, for they rejoice over your success.
