Objection 1. It seems that friendliness is not a special virtue. For the Philosopher says (Ethic. viii, 3) that "the perfect friendship is that which is on account of virtue." Now any virtue is the cause of friendship: "since the good is lovable to all," as Dionysius states (Div. Nom. iv). Therefore friendliness is not a special virtue, but a consequence of every virtue.
Objection 2. Further, the Philosopher says (Ethic. iv, 6) of this kind of friend that he "takes everything in a right manner both from those he loves and from those who are not his friends." Now it seems to pertain to simulation that a person should show signs of friendship to those whom he loves not, and this is incompatible with virtue. Therefore this kind of friendliness is not a virtue.
Objection 3. Further, virtue "observes the mean according as a wise man decides" (Ethic. ii, 6). Now it is written (Ecclesiastes 7:5): "The heart of the wise is where there is mourning, and the heart of fools where there is mirth": wherefore "it belongs to a virtuous man to be most wary of pleasure" (Ethic. ii, 9). Now this kind of friendship, according to the Philosopher (Ethic. iv, 6), "is essentially desirous of sharing pleasures, but fears to give pain." Therefore this kind of friendliness is not a virtue.
On the contrary, The precepts of the law are about acts of virtue. Now it is written (Sirach 4:7): "Make thyself affable to the congregation of the poor." Therefore affability, which is what we mean by friendship, is a special virtue.
I answer that, As stated above (109, 2; I-II, 55, 3), since virtue is directed to good, wherever there is a special kind of good, there must needs be a special kind of virtue. Now good consists in order, as stated above (Question 109, Article 2). And it behooves man to be maintained in a becoming order towards other men as regards their mutual relations with one another, in point of both deeds and words, so that they behave towards one another in a becoming manner. Hence the need of a special virtue that maintains the becomingness of this order: and this virtue is called friendliness.
Reply to Objection 1. The Philosopher speaks of a twofold friendship in his Ethics. One consists chiefly in the affection whereby one man loves another and may result from any virtue. We have stated above, in treating of charity (23, 1, 3, ad 1; Q 25,26), what things belong to this kind of friendship. But he mentions another friendliness, which consists merely in outward words or deeds; this has not the perfect nature of friendship, but bears a certain likeness thereto, in so far as a man behaves in a becoming manner towards those with whom he is in contact.
Reply to Objection 2. Every man is naturally every man's friend by a certain general love; even so it is written (Sirach 13:19) that "every beast loveth its like." This love is signified by signs of friendship, which we show outwardly by words or deeds, even to those who are strangers or unknown to us. Hence there is no dissimulation in this: because we do not show them signs of perfect friendship, for we do not treat strangers with the same intimacy as those who are united to us by special friendship.
Reply to Objection 3. When it is said that "the heart of the wise is where there is mourning" it is not that he may bring sorrow to his neighbor, for the Apostle says (Romans 14:15): "If, because of thy meat, thy brother be grieved, thou walkest not now according to charity": but that he may bring consolation to the sorrowful, according to Sirach 7:38, "Be not wanting in comforting them that weep, and walk with them that mourn." Again, "the heart of fools is where there is mirth," not that they may gladden others, but that they may enjoy others' gladness. Accordingly, it belongs to the wise man to share his pleasures with those among whom he dwells, not lustful pleasures, which virtue shuns, but honest pleasures, according to Psalm 132:1, "Behold how good and how pleasant it is for brethren to dwell together in unity."
Nevertheless, as the Philosopher says (Ethic. iv, 6), for the sake of some good that will result, or in order to avoid some evil, the virtuous man will sometimes not shrink from bringing sorrow to those among whom he lives. Hence the Apostle says (2 Corinthians 7:8): "Although I made you sorrowful by my epistle, I do not repent," and further on (2 Corinthians 7:9), "I am glad; not because you were made sorrowful, but because you were made sorrowful unto repentance." For this reason we should not show a cheerful face to those who are given to sin, in order that we may please them, lest we seem to consent to their sin, and in a way encourage them to sin further. Hence it is written (Sirach 7:26): "Hast thou daughters? Have a care of their body, and show not thy countenance gay towards them."
Source: Summa Theologica (New Advent)