God is a Trinity. That is, relationships of love and friendship are not just an afterthought, or something we need because we're mere human beings. They are the fundamental reality of the universe. There is some mysterious sort of society even within the Godhead. So friendship imitates the mutual love and self-giving of the three persons of the Trinity.
Friendship is therefore a very great good.
Because it is such a great good, its perversion is particularly tragic. That's one of the reasons Augustine is so upset about the theft of the pears. That also explains why he gets so worked up about his youthful unchastity. It got in the way of friendship. He ought to have kept the measure of mind to mind, as he puts it.