Why is she flirting with me?

I have a Co-worker who has been sharing personal things about what she’s going through and opening up to me. She also flirts with me as well…Yes I do flirt back but with limitations. She’s in a relationship currently, she also compliments my heart and has invited me to advent’s she’s going to with mutual friends. (Why would she be doing this if she’s in a relationship)?
Why is she flirting with me?
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