Why are people their own worst enemy?

So often we see that people behave in ways which hinder their own success. We see people being rude to others when the only possible result is to enrage them. We see people fail to co-operate or to help each other and end up with a suboptimum result. Why do people not see that ego can hurt themselves? Are glory and recognition so important that many will sacrifice their own interests for them? We hear that it is better to share in a success than wholly own a failure, yet so many act as though they would rather own the failure.
I see with so many alumni and networking groups that a few people do the work while most others simply enjoy the results. Yet the people who do the work rarely are rewarded. Perhaps by continuing to provide unpaid effort, they are rewarding and training for selfish behavior. I continue to believe that peopl ehave good intentions. It is a pity that they do not see the value of contributing.