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.