The history of heaven travels since way back. Gods were created before everyone. They were the ones who created the universe, using their unimaginable strength and power.
Their powers, as everyone knew, came from the Ancestral God, who was the sole creator of everything.
Despite the legends in the mortal world that the Ancestral God was the only reason why there were humans, it wouldn't actually all be possible without the help of the other gods. All of them contributed and shared their powers, assisting the Ancestral God in doing everything.
When humans were created, the Ancestral God decided to live in seclusion and leave heaven by building his own kingdom. He vowed that he will never interfere with the human and god's affairs unless it was badly needed.
Since then, there was a law that was created about how the gods should not fall in love with humans. The reason for it was that they shall not favor any mortal beings that they have created in this world.