Hmmm, a friend just asked to write something about the theory that every person is acting 100% out of egoism at the core of it all, no matter what it plays out as - the nicest most giving person as much as the most egocentrical one. The theory is intriguing, I admit, but as we go deper and deeper towards the cores of our selves, as with it is physics there are other forces playing a role, thought structures and instincts that we cannot discuss singularly. My process of thought along this line e.g. got me to the question of how time exists; if it is parallel, multidimentional (which would mean every moment in time exists for itself and cannot be severed but experienced in several different realities) or linear and if the human brain has an interdimensional port that can transfer information without the limits of time and physics...way too far.
I don't know enough to actually write something like an answer to that question and I doubt I ever will. I suppose we are not far enough yet in psychologic and physiologic research to determine what motivates us at the core, what egoism actually is and how the human brain develops over the years responding to internal and external stimuli.
What I can say is that I don't think you can compare the egoism that we see in greedy or selfish people to what we are inclined to call egoism of the self, like the sense for self preservation, because one is an instinct we are born with and the other is a pattern of thoughts and emotions in relation to society. So I think it is save to say that egoism has nothing to do with human nature, it is something that people acquire during their lifetime. Everybody has a sense for self preservation, but along with other influences like a sense for community and partnership, a sense for self-progress and, very important, religion or world view and much more we might not see, every human being developes their own logic of how to act and if it makes more sense to be egocentric or gracious and to what level. And other people who went to a similar but completely different process judge those people to determine what they are like, relying on their own experiences...
Things are not always as easy as they seem. Actually barely so at all.