There must be some common ground or area between two persons who form such a relationship. Between people there is enough common ground to form friendships. Every person has experiences of sorrow, joys, hopes, etc. with which to share with another. The inner life of another that is known to God alone becomes to a much less degree open to us through friendship. It partially fills the desire of our incomplete, lonely hearts for completeness in another.
Friendship brings out the best in a person through forgetfulness of self.
Yet it is limited as one cannot give totally of oneself to another because one is not in complete possession of oneself. Friendships are the result of virtue. They are based on a goodness in oneself that another can love. Friends come to discover the good in each other and the relationship grows. When God becomes the object of one's friendship we call it charity.