Because she did something very foolish earlier, like opening up her heart and letting someone in, simply because he knocked nicely on her door.
A petty way to lower her defenses, but a successful one nevertheless, and now her thoughts are invaded with a string of what-ifs, why-nots and oh-fuck-its — delusions, which rotated like planets around moments that reeked of this poison. Like that time when he gave her a book, or sent her a song, or told her stories about pulsing stars.
Now her life is held hostage as she starts to lose her sleep, lose her appetite and lose herself as she drowns beneath the battling tides of her own mind.
Alas, that poor, pathetic girl. She let poison into her heart and now she is dying.