I think because she cared about him and she also wanted to preserve the notes and didn’t want them to get lost in the shuffle or destroyed. She probably needed to make sense of them and maybe needed some distance so instead of throwing them away she sent them to her friend? She was also dating Nate at the time. Maybe she didn’t want him to see it?
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"And you know, you know, you know, this can be beautiful, you say you're numb inside, but I can't agree. So the world's unfair, keep it locked out there. In here it's beautiful."