Why would a girl be looking and smiling at me as she enters the building?

I was sitting at the front counter on phone and she walks in with her friend. As she is walking past me she is smiling and looking at me. I don't know her all that well. She is the only one that really does it.

She does seem to always look at me and smile and even say hello to me.

Why would a girl be looking and smiling at me as she enters the building?
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