What causes a girl to start giving attention to a guy she doesn't know?

She just one day started getting excited to see me when she didn't know me. She would give me a big wave and smile. Now she and i talk so comfortably with each other. It's just happened so naturally. It doesn't feel awkward to talk to her either. But I've noticed she will only have an in depth conversation with me when I'm alone. Otherwise she will still say hello to if I'm with someone.
What causes a girl to start giving attention to a guy she doesn't know?
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