So Shiori is literally saying: "I will only eat you when you don't want me to eat you.".
Shut the fuck up...
Happy people taste better. And it's not like she has ever denied being literally a monster, quite the opposite. The main reason she's here is to raise her food to be the tastiest it can be, the resulting relationship is just the means to do so. Depending on where the story goes from here, it could possibly cause her objective to change down the line, though.