Caster's fascination of humanity began when she left for Izumo. Observing those who worship her, she became curious about the lives of mortals. From a divine being perspective, they're beyond inconsequential like a human view on ants and cockroaches. Humanity and Divinity are always inequality.
As she looks down on mankind, part of her wanted to know more about human condition. Her fixation came from a weird sense of justice. She wanted to use her powers to scold and punish those of evil acts. As she watched humanity, her curiosity grew more intense. To her, humans are depressing and weak little creatures yet they are happy and smile.
Though humans can't see Caster, they still worshiped her with their body and soul. She had an epiphany, humans are devoted to her and she wanted to serve someone. She became driven by curiosity, she desired to be human which was against her divine providence and made an enemy of nature. But it was a mistake, proclaiming herself a delusional child goddess who admired humans too much and should have stayed in the Celestial cave.
Do you want to be the person Tamamo No Mae Caster will adore? Here's a link where you can buy this beautiful dakimakura.