Girls, Why does she smile to herself when I talk to her?

For several months we would stare at each other but never talk.
I noticed she would always stare or watch me. Whether it be in the mirror , walking passed or just in general. But she would never say hi or smile to me. sometimes she would smile to herself if I’m around or if I saw her with guys.

Recently , I spoke to her in the gym for the first time. I only wanted to ask if she was using something and I asked how long she’d be. She kept smiling to herself whilst I was talking to her. Why does she do that?
also sometimes she comes in with a guy, and she keeps staring when he’s not around. Then when he’s around and I walk in she smiles to herself?
Girls, Why does she smile to herself when I talk to her?
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