All or almost all people tend to act selfishly. Trying to change this is foolish.
But this does not mean that everyone tends to act only selfishly. In fact, a certain amount of selflessness can also be found in animal behaviour (biologists have accurately described this in terms of the so-called Hamiltonian principle).
And it certainly doesn’t mean that it’s a good idea to encourage people to be selfish. Numerous studies show that when people are in environments where there is talk of promotion, profit, competition, etc., their behavior changes for the worse. Not only does the level of consideration decrease, but the ability to work rationally together to achieve goals decreases as well.
A social system based on the idea that people will not act selfishly is naive. A social system that systematically reinforces selfishness is destructive.