Originally Posted by
Sesto
When God first left the world he made a wish: "May these days last forever". His wish came true, and he became immortal. This is said in the second episode. Eventually, his wish changed into wanting to die. Since the requirements for his wish (regrets, family, friends, etc) never fulfilled, every time he died he came back to life. When they finally fulfilled, his wish came true, and he died. He came back dead because that is what happens when anybody dies in this world. They can only rest when buried by a gravekeeper.
I haven't read the source material, that's just my guess.