The theme of friendship was prominent among the Greek Fathers, even those who lived in the desert, as Cassian attests in his sixteenth conference. It reached its climax in St. Thomas, who defined charity as friendship with God (IIaIIae, q 23) and who described the work of the Holy Spirit in the world as a work of friendship (Summa contra gentiles IV 21-22.)
This theme has completely disappeared from modern books on morality. The reason is obvious: friendship, being essentially free, could hardly be considered an obligation. Friendship can create obligations, but the inverse is not true. As a result, friendship has been excluded from the field of morality as an indifferent sentiment—mistursted by moral theologians, moreover, because of "particular friendships."
Likewise, we will look in vain for a simple allusion, still less for a full treatment, of the virtue of courage in many of the manuals. Courage is not a matter of obligation. Yet it is numbered among the four cardinal virtues. St. Thomas associated it with the ideal of martyrdom, the inspiration of the early centuries of Christianity. Everyone knows from experience how great is the need for courage throughout our moral life.
It is easy to reinstate friendship and courage in moral theology if we begin with the question of happiness. Can a person be happy without the harmonious relationship we call friendship? It is a concrete form of charity. Again, how can we be happy without the courage that strengthens us in the face of difficulties and keeps us steady in the day-to-day grind? This is all the more true when our goal surpasses human power and calls for an audacious faith and trust in the Word of God.
— Servais Pinckaers, The Sources of Christian Ethics