Why do men not care about many things?

So.. .. .. my girlfriend has a male friend and they have been knowing each other since a long time, almost for 7-8 years. Recently her friend confessed his feelings to my girlfriend saying I love you even while knowing that she is in a relationship with me! I confronted my girlfriend and she spoke to him and the air is cleared. . . But I can't stand the fact that he is still around. . . And I don't think I can even in future! What to do?
Why do men not care about many things?
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