A friendship of bad men becomes evil, for they engage in bad pursuits, and they become more evil by their mutual influence on one another. The friendship of good men is good, and friends become better through their good influence on one another and by correcting each other. Aristotle's discussion of friendship implies his fundamental view of human beings as social beings. Friendship is thus a necessity for the good life in Aristotle's view. While Aristotle does discuss friendships based on usefulness or on pleasure, he does not consider these to be friendship in the genuine sense.
True friendship is based on virtue, and requires wishing the good for the other.
Wishing the good for a person in the Aristotelian sense is not at all a vague and sentimental concept. Primarily, wishing the good for someone means helping him to be virtuous, for it is by being virtuous that a person will be genuinely happy, as established in Book One. For this reason, friendship is what many natural law theorists call a genuine human good, something in human life which it is natural for people to seek because it fulfills an intrinsic human need and is an end in itself. As relational beings, people need genuine friends in order to live a fulfilled human life. Friendship is a sort of self-love.