Why would this girl suddenly be nice to me?

Its not like she was ever mean or anything, but now she is so nice and easy to talk to. Before I had to kind of struggle to get a word out of her and everything we said to each other was always laced with sarcasm and we would tease each other a lot and she would get annoyed pretty fast when I talked to her.

All of a sudden, she is nice and talkative, laughs at my jokes and we can have nice conversations ( we did before, but it feels different now) now she smiles a lot, giggles and it makes me feel "good" when she smiles and looks into my eyes (before it was more like a glare lol) althougj she does look away when I make her smile.

Its noticeable and I asked her why and she didn't want to tell me or talk about it, what could the reason be?

Why would this girl suddenly be nice to me?
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