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Yeah I also want to know the reason, perhaps anyone know?

Here's the answer: She was having nightmares about Marisa stealing her heart and she (Alice) would go into a love frenzy and everything would turn all red. So Alice bought the Butterfly Dream Pill to have pleasant dreams about being a butterfly instead of horrifying nightmares about Marisa stealing her heart. Eventually she got trapped in the dream and Eirin had to rescue her. 

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