Girl's Behavior

Girls, How come when I did what I said I would do she now views me as her best friend?

I told her that I would pay for something and then she said “remember what you said to me when you said you’d pay for it” (I promised her earlier that I would pay for it a few days earlier) and then I promptly paid for it.

She was shocked that I actually did it. She then said you are my best friend and she’s been really different towards me. Like she’s so happy over and over.

She is younger than me like 23 and I’ve flirted with her before but I’m wondering what is going on like why would she say we are best friends after I did this. Why would she be like this?
My friends say she does like me but I’m wondering why this is such a big deal for her?

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She does like you
She’s been hurt before
She values your honesty
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Girls, How come when I did what I said I would do she now views me as her best friend?
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