Because you're creeping her out.
It might not even be you, precisely-- just the fact that you're 15 and she's 12. Most likely not even old enough to be interested...
...if she weren't from a super-religious family and not allowed to talk to boys.
That might be the whole reason, and not anything particularly about you.
Regardless-- Dude, you're 15. She's 12. That's enough to get you charged with a crime for so much as putting a hand on her shoulder.
Direct your eyes elsewhere, at least for the next six years.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"