What does it mean when girl shows lot of interest and reject’s when I start’s showing her interest?

I had a coworker who literally ask me lot of personal questions and goes extra miles and helps me. Suddenly when I confessed her about my interest she seems me as a friend, WTF I have many friends I don’t do all stupid stuff, why it so complicated just to understand a girl.
What does it mean when girl shows lot of interest and reject’s when I start’s showing her interest?
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