thats true, in the sense that there always is a self, but all the point of the word selfishness is that of defining what "self" is. that is to say, selfishness is not defined as "something that happens for a causal cause", because that is always the case for anything, even for matter which is not commonly described as selfish. the word selfishness really referrers to the narrow self of a single perspective, that is even less that a single person, cause we know, we hurt our-selves for selfishness all the time.yet, the sharing of the beer makes YOU happy
if you are happy for a tactical reason though, you are not truly happy, but if you are truly happy because you enjoy common happiness, than happiness itself rules both you and the mate, so happiness it self becomes the common self, the broader self, even the nondual self.
and its not defined as non dual, because of a metaphysical assumption about an energetic link between two people, and not because of the assumption of a direct causal link, ie one happiness triggers the other happiness, it is defined like that, because of how happiness it self works, generally or abstractly. it just works TM.
cause that is human nature. so the selflessness of joined happiness is the ultimate product of the function of every single happiness. happiness strives to nonduality, because that way, it is just bigger. its only happiness, if you share it. thats the programming of heartfelt happiness.
its irrelevant who of two people smiled first, because what was actually first, was human nature, the heart, which programmed them, to find, that joined smiling is more happy than smiling alone. so that is the actual causality, that embraces the striving for nonduality everywhere in the relative world, especially in human nature. its hunting you down, and then it kills you.