What is "me'Rachok"?
Rashi: It is in the merit of the Avos.
Malbim: Hashem will appear to them from afar. Even though He will stand afar, and the Hashgachah will be wrapped in hidden miracles, they will see Him from afar and recognize that He supervises over them. I.e. He will lighten the yoke of exile on them and they will find rest.
Who is "Li", and who said "Ahavas Olam Ahavtich"?
Rashi: I, the Navi; He told me to tell Yisrael "I had eternal love for you."
Radak: Keneses Yisrael, in exile, replies to Hashem's words "Matza Chen ba'Midbar" (verse 1). Yisrael says, true, Hashem appeared to me a long time ago, but now He hid His face from me. Hashem replies, My love for you was not temporary - it is eternal!
Malbim: Yisrael say, Hashem appears to me from afar. Hashem says, I had eternal love for you.
Why does it say "Al Ken Meshachtich Chesed"?
Radak: Because I had eternal love for you, I drew My Chesed on you, and I appeared to you in My honor on Har Sinai. Even though I abandoned you for many years in exile, I will build you, and you will be built (verse 3).
Malbim: The old nuptial love will be aroused through the grace they found in the Midbar, i.e. the length of exile and what they suffered for the honor of His name. Hashem says, via this love I drew you from exile with Midas ha'Chesed. He will redeem them and be to them Elokim, in His Chesed.
