Aristotle offers us no sustained account of civic friendship (πολιτική φιλία), only remarks scattered throughout the Nicomachean Ethics and Eudemian Ethics. In this paper I hope to make clear what his views on civic friendship are.
Citizens will feel affection for one another due to the mutual benefit they receive from living together in a city. They agree about what is advantageous for the city: who should rule, how the city should be run, etc.; and to the extent that they care about the common good, they all have one aim. In addition, the affection a citizen feels for his fellow-citizens most likely involves a concern for their moral character (and its improvement) . For how well one is able to support the city and the constitution, is likely to be loyal and not a traitor, is willing openly, honestly, and intelligently to discuss issues in the Assembly and the Council, is likely to be fair in the marketplace, etc. depends on moral character.
It is the awareness of a common aim and the awareness that everyone accepts it and is working to achieve it—each thereby benefiting himself—manifested in and encouraged by their 'living together,' that most of all holds a city together.
Mayhew, Robert, "Aristotle on Civic Friendship" (1996). The Society for Ancient Greek Philosophy Newsletter. 197.