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A vampire basically having to be an amazing Caretaker to their blood source because human blood tastes so much better (and maybe more nutritious just for fun) if the human is well fed, happy, calm, ect. Maybe the human wants to stay with the vampire, because honestly this is so much better than their life used to be. They don't have to stress about anything, the food's the best they've ever eaten and they barely ever have to feel scared or sad anymore.
Bonus: The human had a condition that made it much more difficult for them to live life before, whether mental or physical.