According to the theory of Adam Smith, the world’s famous economist, self-interest is defined as acting in the way that is most personally beneficial. He famously explained that it is possible to achieve the best economic benefit for all even when, and in fact because, individuals tend to act in their own self interest. Economists assert that self-interest explains virtually all behavior. They believe that it is an important human motive, and it has well-established explanatory power. In fact, most of the behaviors around us are the result of self-interested behavior. 

So why does the chef choose to cook? The reason is self-interest. Sometimes self-interest produces a behavior that benefits others. The chef wants to earn enough money in order to feed his family and buy things he wants and the most effective way he discovered to do this is to cook for you. In fact the food he cooked has to be delicious enough that you are willing to give up your money for his food. The chef does, not only, serve his self-interest, but also, produced a good that is valuable for you. 

However, there are some situation which the self-interest model’s prediction fall short as well. For example, some people vote in presidential election even though they already know that no single individual’s vote has ever made a difference. We anonymously donate out money to charity. Also, most Americans leave tops in the restaurant even though they will never visit that restaurant again. From society’s viewpoint, rather than acting in a purely selfish manners, forgoing self-interest in such instances leads to a better outcome.
Astonishingly, in an experimental study of private contributions to a common project, two sociologists from the University of Wisconsin, Gerald Mar and Ruth Ames, found that first-year graduate students in economics contributed an average of less than half the amount contributed by students from other disciplines. Additionally, in one experiment, the cooperation rates of economics students fell short of those not studying economics, and the difference grew the longer the students had been studying their respective subjects. Does this mean that repeated exposure to the self-interest models makes selfish behavior more likely? 
In my opinion, it is not that economists are wrong to emphasis on the important of self-interest. But those who assert that it is the only important human incentives are missing something important in their life.