In my headcannon, Galactus fights Apocalypse to help the heroes who saved him hence his life debt. I picture this quote:
Apocalypse; You vow madness. You are the last of your kind. I was the first of mine; the future dominant species of all worlds!
Galactus: You only speak half a truth. For in all of the worlds, I am unique. And I provide balance which you endanger. I am Galactus, the be all and end all of the universe.
Galactus: You only speak half a truth. For in all of the worlds, I am unique. And I provide balance which you endanger. I am Galactus, the be all and end all of the universe.