In order to answer these questions, it is necessary to see what makes something likeable. The three possibilities are goodness, pleasure and usefulness. To be friends, two people need to be well-disposed toward one another and wish each other's good, and they must know that this is the case. In friendship based on usefulness or pleasure, the person is not liked in himself but because of the good or pleasure he can provide. Such friendships are easily dissolved.
Perfect friendship exists between good men who are alike in their virtuousness.
Wishing a friend good for his own sake is the highest degree of friendship. These friendships tend to be longlasting because virtue is something stable. Because both friends are good their friendship is both beneficial and pleasant as well, and thus they unite all the three reasons for friendship. Such friendships are rare because few people have the capability for this sort of friendship and they require time and familiarity to form. Friendships based on pleasure can vary in their duration.