Why is everyone so selfish?

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So I'm just wondering why people are so selfish? I have tried to invite many of my friends here, and seen few of them registered here, but everyone had removed the referal and i havn't gained anything. Do they gain anything by making me not gain anything? They are my friends and I really don't understand why would they want to do something like this...I asked one of them why did he do that, and he just said he didn't take it off...It really makes me sad and angry at the same time, and I wonder if anyone had the same problem as i do?