In The Inner Citadel, Pierre Hadot analyzes the Stoic rationale for valuing altruism.
Human beings participate in Reason - Reason is the defining characteristic of humanity, shared only with the gods, if such exist. In fact, many ancient Stoics believed that the logical faculty within a human being was in fact a spark of the Divine within us, essentially a small god, or a "piece" of God. Hence, "Intelligence and reason are common to all reasonable beings, for by virtue of their universality which transcends individuals, they allow us to shift from the egocentric viewpoint of the individual to the universal perspective of the All. This is why intelligence and reason tend naturally to envisage the good and interests of the Whole. Logikon ("rational") and koinōnikon ("caring about the common welfare") are inseparable (VII, 55)"
Hadot goes on to analyze the Stoic view of egotism, which is that it is harmful to the individual as well as for the whole. The oft-repeated maxim that "Universal Nature has made rational beings for the sake of one another" leads to the conclusion that a person who separates his own interests from those of the human race is like a severed body part - useless to itself and the whole of which it was part.