Because helping others provides both psychological benefits and strengthens your social network. If one thing is evident about the human experience, it is that strong social networks ensure survival. Thus, it is to everyone's personal advantage to create a strong social network, and this is done through generosity. Even the most tyrannical dictator needs to have an inner circle with whom he is generous. When he fails to appropriately reward his inner circle, the tyrant is deposed, assassinated. This realization has made me far more generous than did my old fear of hell.