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.
Good self-love consists in wanting what is genuinely good for oneself. It is a friendship between the highest part of the soul and the lower parts, in which the highest the lower parts of the soul are brought into harmony with the intellect, Aristotle sees as the most proper identification of a person. A friend is another self, and thus in genuine friendship one wishes the good for another and helps him to achieve that good, which consists primarily in being virtuously. A friend is another self, and as such a genuine friend is indispensable for the attainment of self-knowledge. Self-knowledge is indispensable for growth in virtue, and genuine friends provides a person with a sort of mirror with which to see himself more clearly.