Although this song appears to be a standard love song, it was Gershwin's final composition. Ira wrote the lyrics after the death of George. More than anything it was about a brother's undying love. How could anyone listen to this song without being brought to tears?
Think of "our love" as the Love that is God/Being. It's stable, everlasting. Material things will of course disappear; matter doesn't last. Love, Truth does.
George and Ira were geniuses.