What is the role of self-interest in economics? Give an example

Self- interest is an assumed attribute of people in the society while studying economics. It means that the subject assumed that people seek their personal interest while conducting trade or getting engaged in a production or consumption process. The customers in the field are said to be rational. It is believed that they will always choose to do those things that maximizes their benefits and satisfaction level. For example, a customer will always prefer that good which gives him/her largest possible utility and is low priced as well.