We are all selfish. We all have a self, kind of. We are self-ish. We are so selfish that the Buddhist idea of no-self is hard to understand. It is worth trying to understand because the idea of a separate self is what causes us so much distress. To see how your idea of yourself causes you problems, just think of any of your problems and imagine them happening to somebody else. If your best friend is having a problem, you are able to help them through it because you have a different perspective on it. It is a manageable problem to you because it is not happening to you. It is happening to your friend, to your non-self. The difference between how you feel about your friend’s problem and your own problem is your selfishness.
The idea that your self is just an idea comes from looking at existence as a whole. Everything is interconnected with everything else. We are so interconnected with each other that the idea that we are something separate is inaccurate. Although we have a self which is what we have always experienced as us, we also have a non-self, which is how we fit into the big picture. As we go between our self and non-self, we are self-ish. When we recognize our interconnectedness is is easier to be selfless because we can see our friends, family and enemies as extensions of our self. If your self includes your best friend, then helping your friend is selfish in a fantastic way.