Aristotle studies happiness, and says that happy men necessarily have friends. That's why, when he talks about happiness, he also talks about friendship.
The author  tells us that love is the virtue of those who are friends and this feeling is experienced by what men want, that is what is kind, or to say what is possible to love: what is good, what is useful or what is nice.
And it's from what is kind where the different types of friendship are born. So there could be feelings of friendship for interest, for pleasure, or because of the virtue of the friend.
Aristotle when he talks about friendship makes a difference between good men and bad men, the friendship between good men is given by the virtue that exists between the friends and as the possibility of offense between good men does not exist the friendship between this type of men is stable, on the other hand among perverse men friendship is fundamentally for pleasure, the perverse people tolerate each other mutually until they lose continence, Aristotle tells us, remember that the perverse people have fundamentally multiple personalities, that is, they can act  by passion ( impious) or by reason (sinners).
All these types of friendship needs of equality and that because friends wants and receive from each other and this is where the concept of justice is related to friendship.
Higher is the friendship, worst is the injustice, or to say where there is no justice among men it is no possible the friendship. More is the injustice, tells us Aristotle, friendship is given in a lesser degree.
It is because of this that the foundation of justice and friendship is love. From the good governments depends the good administration of justice and therefore the social friendship among men. Men naturally seek happiness and this last concept is achieved more in the "city" or community when is reached the common good. The common good ensures friendship and equality between men and that is the end of life in community. The selfishness and impiety, typical of tyrannies, are against the dignity of men. Without the common good is not possible the equality and friendship between people. Common good, justice and friendship are in some way synonymous in the Aristotelian thought.