So she's obsessed with him because he's pretty much the only one in the entire world that can actually care about her and show her kindness.
Not so much kindness but probably acknowledging her existence at all. Her yandere vibes give me the feeling she is a normal sentient being with need of community and feeling of belonging but due to her condition therefore absolutely attention starved.
But her explanation of dusk elves raises multiple questions: couldn't she stay with other dusk elves as well? If not, how do dusk elves reproduce? Why was she even caught in the first place, if nobody can consciously observe her?