In his epistle, De Regno, St. Thomas provides various arguments for this position which are worth listing here due to their simplicity and coherence.
i. Man is physically unsuited for survival in isolation. “For all other animals, nature has prepared food, hair as a covering, teeth, horns, claws as means of defence or at least speed in flight, while man alone was made without any natural provisions for these things. Instead of all these, man was endowed with reason, by the use of which he could procure all these things for himself by the work of his hands. Now, one man alone is not able to procure them all for himself, for one man could not sufficiently provide for life, unassisted. It is therefore natural that man should live in the society of many.”
ii. Man lacks the sufficiency of instinct found in other creatures. “All other animals are able to, discern, by inborn skill, what is useful and what is injurious, even as the sheep naturally regards the wolf as his enemy. Some animals also recognize by natural skill certain medicinal herbs and other things necessary for their life. Man, on the contrary, has a natural knowledge of the things which are essential for his life only in a general fashion, inasmuch as he is able to attain knowledge of the particular things necessary for human life by reasoning from natural principles. But it is not possible for one man to arrive at a knowledge of all these things by his own individual reason. It is therefore necessary for man to live in a multitude so that each one may assist his fellows, and different men may be occupied in seeking, by their reason, to make different discoveries—one, for example, in medicine, one in this and another in that.”
iii. Through the gift of speech man is made for communication. “…the use of speech is a prerogative proper to man. By this means, one man is able fully to express his conceptions to others. Other animals, it is true, express their feelings to one another in a general way, as a dog may express anger by barking and other animals give vent to other feelings in various fashions. But man communicates with his kind more completely than any other animal known to be gregarious, such as the crane, the ant or the bee. With this in mind, Solomon says: ‘It is better that there be two than one; for they have the advantage of their company.’ ”
 DR, 5.
 DR, 6.
 DR, 7.