How can someone forget other person so easily?

My brother really liked one girl. She liked him for a long time. Even her friends told me she likes him so i could push him into something. He asked her out and she said yes. They kissed few times and then finally went out. But then she turned him down. He doesn't know why. Now they say hi and dont talk more. He is really hurt. He looks like she took life away from him. I wonder how someone can forget person they liked for such a long time so easily and just like that. Are we so easy forget. I feel really sorry for him.

How can someone forget other person so easily?
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