The drug theory is pretty creepy...whether or not that's the case, looking at it from just a girl-boy perspective, I don't think it's possessive, just really sweet. The "You don't need other friends anymore" isn't a threat, it's a statement to describe what she feels at that moment - at that moment, all she feels like doing is being with him, like children almost, happy. "Your best friend I've come to be" - they've become really close and she doesn't want him to leave, but she's not tying him down or anything, it's just the way they feel, it's sweet.
The drug theory is pretty creepy...whether or not that's the case, looking at it from just a girl-boy perspective, I don't think it's possessive, just really sweet. The "You don't need other friends anymore" isn't a threat, it's a statement to describe what she feels at that moment - at that moment, all she feels like doing is being with him, like children almost, happy. "Your best friend I've come to be" - they've become really close and she doesn't want him to leave, but she's not tying him down or anything, it's just the way they feel, it's sweet.