A memorable passage from Epictetus in The Discourses.
Be not deceived, every animal is attached to nothing so much as to its own interest. Whatever then seems to hinder his way to this, be it a brother or a father or a child, the object of his passion or his own lover, he hates him, guards against him, curses him. For his nature is to love nothing so much as his own interest; this is his father and brother and kinsfolk and country and god. At any rate, when the gods seem to hinder us in regard to this we revile even the gods and overthrow their statues and set fire to their temples, as Alexander ordered the shrines of Asclepius to be burnt when the object of his passion died. Therefore if interest, religion and honour, country, parents and friends are set in the same scale, then all are safe; but if interest is in one scale, and in the other friends and country and kindred and justice itself, all these are weighed down by interest and disappear. For the creature must needs incline to that side where ‘I’ and ‘mine’ are; if they are in the flesh, the ruling power must be there; if in the will, it must be there; if in external things, it must be there.
If then I identify myself with my will, then and only then shall I be a friend and son and father in the true sense. For this will be my interest—to guard my character for good faith, honour, forbearance, self-control, and service of others, to maintain my relations with others. But if I separate myself from what is noble, then Epicurus’ statement is confirmed, which declares that ‘there is no such thing as the noble or at best it is but the creature of opinion’.