Girls, why she keep smiling to herself and staring at me?

For a while a girl in my gym always stares at me she walks past when I’m on the exercise bike. If she walks in and sees me she just stares. If I train in the weights room she stops In between her exercises and just watches me , then if I walk away she stares at me in the mirror.

However , one time she came in with a guy (think he’s her boyfriend). Then she started smiling to herself. Once when he was beside me I play American football so very big and lift weights for a long time.

Then she walked In front of me and put some weights back I was waiting for her to finish and started smiling to herself her boyfriend caught her and he seemed annoyed with her.
Girls, why she keep smiling to herself and staring at me?
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