Selfish gene and altruism

What is a perfect world? I have always hope for a world where people can be selfless, help everyone with all their power. However, the world is not created this way. Natural selection has favor the selfish genes as it provide these individual with higher survival chances. This lead me to think if i should feel upset about my sister's choice. I hope she will not be the reason for the failure of a new startup. Her decision for leaving a startup at its critical moment is a display of selfishness. However, since this is what human's have evolved to become, I don't know if I can hold her responsible for it. Life is difficult.