Why does a girl that rejected me get angry when I found someone new?

She liked me first but then she rejected me. We kissed few times and went out. Now i met a new girl and we are in takking stage and the girl that rejected me knows about it and she got angry i would say. She has always questions like wheres that your new girl or what about that blonde girl of yours and she says it always with a stupid tone.

Why does a girl that rejected me get angry when I found someone new?
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