Freud called our concept of the self the ego, and it was meant as a representation of how we feel about ourselves. We tend to perceive ourselves differently from those around us, and that perception fuels much of our action. Rational egoism argues that self-preservation is the normative state for humans and concepts such as altruism go against our nature.
Pleasure and Pain
Consider that someone was confronted with two choices, one that would bring a pleasurable sensation and another that created a painful sensation. Almost everyone without question would choose the pleasurable sensation, if they were conscious of the outcome ahead of time.
This is rational egoism at work. It’s an instinct for self-preservation, yes, but it’s also the very basis of human nature. If those choices were altered slightly, such that the pleasurable sensation would be given to someone else while a painful sensation was felt by the subject, the outcome of the experiment might be very different.
Some of that is shaped by human experience, but this is ingrained in children from birth. You need to teach children to share, after all, because they would otherwise hoard for themselves.
Taken to the extreme, rational egoism can lead to incorrect thinking. For example, saving for the future. Rational egoism might argue in its most extreme position that a future self is different from a present self, and thus saving goes against self-interest.