Why would a guy hate to core the girl who rejected him despite several years?

The guy was rejected by the girl several years back. Now they met and he acts like he never know her name or existence. And they both are single.

Yet he feels bad when she hangs out with other boys.

by the way the girl is question now looks far amazing than many and has a bunch of guys who are willing to be with her.

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+1 y
He is my friend and used to love her the most. From what I have seen, he worked out of his way to make her notice him. and the way he is now acting towards her makes me doubt his feeling ever towards her were genuine.
Why would a guy hate to core the girl who rejected him despite several years?
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