Trick Slattery, George Ortega, and Jamie Soden talk about the hedonic imperative or pleasure principle. We seek pleasure and avoid pain. This is just one of many reasons we don't have a free will. People keep doing what feels good(pleasure) and try to avoid what feels bad(pain). Upon this is our basis for morality. Some people derive pleasure from doing what they feel is right for someone else even when it may hurt them at the moment. So even altruism falls under this category.