Why do girls sometimes act cold towards guys they like?

I am not generalizing here. I am perfectly aware that not every girl is like that, I just tend to see often how girls try to act cold, and uninterested whether in a relationship with boyfriends, or with guys they like. They will say to him that they don't like him at all, make him jealous, make him do all the work and show to her that she means something to him, and yet again not doing the same in return... What gives? Is it just fear of being hurt, or something more?
Why do girls sometimes act cold towards guys they like?
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