Hello @lukestokes
For the most part of human behaviour, people usually are self-centered and selfish. But we can’t simply claim that altruism is an act of selfeshness as selfeshness implies not caring about the wel-being of others in request of personal benefits. While altruism is caring for others for personal moral benefit as this post claims. This claim might emphasize on selfshness, or we might say that there is benign selfeshness and malignant selfeshness.
If every human act is eventually leaeds to one’s own well being, why are there empathetic individuals who suffer the feelings of others?
Selfishness is different than self-interested. One specifically excludes caring about others while the other recognizes we can only help others after we help ourselves and when we help others we also help ourselves.
As to your question, I think that's based on how different people have different mirror neuron function. Those empathetic people will act on behalf of others even more than most to spare themselves negative feelings (at least, that's how the theory goes).