economics assumes that people are selfish?

1 Short Paragraph of discussion needed

One of the key concepts of economics necessarily deals with
constrained choices that people make, based on the process of subjectively
weighing the perceived costs and perceived benefits of future possible actions.

If people are always weighing costs against benefits, and
making choices that presumably benefit themselves, does this mean that
economics assumes that people are selfish? What is the relationship between
selfishness and economic reasoning?