Once upon a time, there was a little paingod attached to humanity. It was mostly benign and useful; it was there to warn people not to step into that patch of thorns, that that spearpoint was sharp, that fire was hot, that you’d regret drinking that whole skin of beer in the morning. It brought with it the gift of empathy and forethought, as well, so the people were mainly well-served by its presence.
Humanity prospered and grew, and the paingod got ever busier; as societies got larger, not only were there more people experiencing or avoiding pain, but the complexity of their lives created new opportunities for pain. New diseases erupted in the denser populations, wars flared up between competing city-states, social stratification created breeding grounds for envy and contempt. The paingod also grew.
Now this is the thing about gods of all types: they are ambitious. They all aspire to be the greatest manifestation of their gift that they can be. After all, people had been defining god as “that than which nothing greater can be conceived”, so there was definitely pressure to escalate — if it wasn’t the greatest of all possible paingods, then it really wasn’t a god at all, now was it?