Why are some people so selfish?

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A recently divorced friend of mine called earlier today. Her husband had his christmas with her two boys. He claims that he does not have any money to help her pay her part of the boys sports fees, according to the divorce decree they split extracurriculars 50/50. The boys came home with new air jordan shoes, all new warm up gear for basketball, a new DVD player and TV for their room at their dad's, along with dozens of new DVD's. She has already sat down and talked with her boys and explained that there will not be much under the tree at their house this year because she just doesn't have the money to buy a lot. She was really upset, that her ex is trying to one up her, and then claiming he has no money. I guess I am not in that situation, but doesn't he realize that he is just hurting his children.