Why does it say "Al Yiten Lamot Raglecha"?
Shocher Tov 118:9: So Ru'ach ha'Kodesh answered David (when he went to fight Galyus, and said "me'Ayin Yavo Ezri") (1).
Radak: Everyone says so to his friend in Galus, to console him. It is good that you put your trust in Hashem that He will help you. Even though Galus extended, He will not let you falter.
Malbim: The verse depicts that he hears a voice - why do you put your eyes looking for help, as if it is a new matter. Know that His Hashgachah did not veer from you for a moment! It stops your feet from faltering, and protects you from mishaps from outside.
Why does it say "Al Yanum Shomerecha"?
Radak: In Galus, He resembles one who sleeps - "Urah Lamah Sishan Hashem" (44:24).
Malbim: All other guardians tire, and their Shemirah ceases when they sleep.
