First, everyone has an instinctive desire to seek fame and gain. From the perspective of human nature, interest is what human nature wants and what people want to have. Mercenarism is nature, and nature is natural instinct. Interest is the most direct and substantial result. On the other hand, if people are not profitable, they will not make progress and society will stop developing.
Second, everyone has the instinct to serve everyone. People are individuals, and serving the collective can obtain real benefits (material) and virtual benefits (spirit). But you can't have it both ways. If you want to make false profits, you must give up your real profits. For example, Lei Feng, a special figure in a special historical background, has epoch-making significance in his spirit. Lei Feng lives in an era of extreme material scarcity and high spirits. What people pursue is to realize the great ideal of capitalism, to pursue honor, to work hard, to oppose selfishness, and to approve of "self-cultivation". Now the material is rich and the spirit is lacking. Why? Still more realistic and more profitable.
Conclusion: No matter whether people are seeking truth from facts or getting benefits in vain, they will get nothing without paying. Helping others is not necessarily self-interest, and self-interest is not necessarily beneficial to others. It's that simple.