Does only Hashem bless a Tzadik?
Malbim: His Brachah is via Hashgachah, and not natural. His Brachah makes wealthy, and does not make sad. In a natural Brachah, the owner is sad, for he must guard his property and protect it. Guarding accompanies Hashem's Brachah - "Yevarechecha Hashem v'Yishmerecha" (Bamidbar 6:24).
What is the meaning of "ka'Tzinah Retzon Taterenu"?
Rashi: Contentment will surround us like a shield that surrounds a person on three sides. This is like "v'Sha'ul va'Anashav Oterim El David v'El Anashav" (Shmuel I, 23:26).
Radak: Your desire will surround a Tzadik like a shield that surrounds a person.
Malbim: He will be satiated with Your desire that surrounds him like a shield to guard him from any mishap.
