From Summa Theologiae, Ia-IIae, question 4, article 8:
Objection 1. It would seem that friends are necessary for Happiness. For future Happiness is frequently designated by Scripture under the name of “glory.” But glory consists in man’s good being brought to the notice of many. Therefore the fellowship of friends is necessary for Happiness.
Objection 2. Further, Boethius [Seneca, Ep. 6] says that “there is no delight in possessing any good whatever, without someone to share it with us.” But delight is necessary for Happiness. Therefore fellowship of friends is also necessary.
Objection 3. Further, charity is perfected in Happiness. But charity includes the love of God and of our neighbor. Therefore it seems that fellowship of friends is necessary for Happiness.
On the contrary, It is written (Wisdom 7:11): “All good things came to me together with her,” i.e. with divine wisdom, which consists in contemplating God. Consequently nothing else is necessary for Happiness.
I answer that, If we speak of the happiness of this life, the happy man needs friends, as the Philosopher says (Ethic. ix, 9), not, indeed, to make use of them, since he suffices himself; nor to delight in them, since he possesses perfect delight in the operation of virtue; but for the purpose of a good operation, viz. that he may do good to them; that he may delight in seeing them do good; and again that he may be helped by them in his good work. For in order that man may do well, whether in the works of the active life, or in those of the contemplative life, he needs the fellowship of friends.
But if we speak of perfect Happiness which will be in our heavenly Fatherland, the fellowship of friends is not essential to Happiness; since man has the entire fulness of his perfection in God. But the fellowship of friends conduces to the well-being of Happiness. Hence Augustine says (Gen. ad lit. viii, 25) that “the spiritual creatures receive no other interior aid to happiness than the eternity, truth, and charity of the Creator. But if they can be said to be helped from without, perhaps it is only by this that they see one another and rejoice in God, at their fellowship.”
Reply to Objection 1. That glory which is essential to Happiness, is that which man has, not with man but with God.
Reply to Objection 2. This saying is to be understood of the possession of good that does not fully satisfy. This does not apply to the question under consideration; because man possesses in God a sufficiency of every good.
Reply to Objection 3. Perfection of charity is essential to Happiness, as to the love of God, but not as to the love of our neighbor. Wherefore if there were but one soul enjoying God, it would be happy, though having no neighbor to love. But supposing one neighbor to be there, love of him results from perfect love of God. Consequently, friendship is, as it were, concomitant with perfect Happiness.