Why is a girl who rejected me mad that I was talking to another girl, if she didn’t want to be with me?

So there was this girl I was in love with for over a year, who kept rejecting me. So finally I decided instead of going nuts about her I will move on, and talk to other girls who were actually interested in me.

So today at work, another girl was flirting with me and my old crush was furious, and giving me a dirty look. Why would she care if she didn’t want me before? People say I’m a really good looking guy, but my old crush is too if that matters. Why would she get mad another girl is talking to me?
Why is a girl who rejected me mad that I was talking to another girl, if she didn’t want to be with me?
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