St. Aelred of Rievaulx was once told by St. Bernard of Clairvaux (a character in the Commedia) to write what he knew about spiritual friendship. His works on the subject became plentiful; chief among them being Spiritual Friendship. Before laying down a general exposition of this great work, it is necessary to clear something up. It is interesting that modern 'scholars' invoke the name of St. Aelred as a medieval champion of homosexuality. It is necessary, first, to clear his name. The following is the quote that is so often taken out of context:
"While I was still a schoolboy, the charm of my friends greatly captivated me, so that among the foibles and failings with which that age is fraught, my mind surrendered itself completely to emotion and devoted itself to
love. Nothing seemed sweeter or nicer or more worthwhile than to love and be loved."
Now, if these so-called scholars were anything of the sort, they would have understood this quote in the whole context of Aelred's philosophical dialogue, Spiritual Friendship. Perhaps if this were done, there wouldn't be so much misconstrued about his work. If one looks at the above passage, Aelred is simply referring to a tendency in humanity, present only because of concupiscence, to love being loved. This is just a misplacement of the proper object of love which is and must be Jesus Christ.
Moving on, then, to his work. Aelred begins early on by exposing a general concept of Love which obligates us to love one another and treat each other with respect and gentleness with a minimal aspect of tolerance. He says that friendship surpasses this as a fulfillment of love. It is this concept of Charity that will lead into Thomas' treatment of friendship in a specified concept of friendship as love. "In friendship, there is nothing dishonorable, nothing deceptive, nothing feigned; whatever there is, is holy, voluntary, and true." "Friendship shines forth." So far, there is nothing that Aristotle would not have said or did not, in fact, say.
The great leap in the concept comes in the following, famous quote: "And thus, friend cleaving to friend in the spirit of Christ, is made with Christ but one heart and one soul and so mounting aloft through degrees of love to frien
dship with Christ, he is made one spirit with him in one kiss." The two great truths expressed in this quote involve friendship centered and directed toward Christ and the concept of a kiss which is not necessarily of the flesh.
The one thing the virtuous pagans were missing in their concept of friendship was the proper end to which it should be directed. The purpose of friendship for a Christian is the same as the purpose for any event or experience: to get to heaven. Consider this analogy: two friends do not sit at a table looking at one another for truth, they walk along the narrow path both facing the same direction: toward Christ. As they walk facing Christ, they are there to help bear each other's cross and baggage and they can even help one another when they fall. This is the way God has designed it: that we may help one another in community on our way to heaven. This is the concept of Spiritual Friendship that Aelred is putting forth.
He is famous for his explanation of this term: "kiss." This is perhaps where many go wrong thinking mistakenly that he is expressing sympathy for a certain orientation. He is, in fact, careful to note that he is not speaking, necessarily, of a 'fleshy' kiss. He is speaking more in terms of a metaphor as if two souls are bound in friendship by a kiss. Aelred does point out that a physical kiss of peace or greeting is also acceptable and perhaps laudable but also specifically says that "the perverse and lustful strive to give relish to their shameful acts even with this good which the natural law has instituted to signify the things we have indicated, defiling this very kiss with such shame that thus to be kissed is nothing else than to be corrupted."

Then he moves to the spiritual kiss which is marked "not by a meeting of lips but by a mingling of spirits, by the purification of all things in the Spirit of God." He compares this meeting of souls to the breath of Christ on his apostles which is bound up with the Holy Spirit. So the spiritual kiss is almost a union of two friends by the Holy Spirit whose union aims always toward Christ and their salvation.
Finally, Aelred reiterates the most important point and most pertinent to our discussion. In his dialogue with two youths, one youth named Walter says that he thinks it would be better and less confusing to have no friends at all in this life. Aelred says that this is absurd since happiness is unattainable without friends. Then he gives the same example as Aristotle!!! He asks his listener to try and imagine that he were the only one on earth with every treasure and possession imaginable. Then he asks him to imagine the value of all his possessions with no friends, then with another who cannot speak or relate to him, and finally to share these possessions with a friend. They all come to the conclusion that friends have an effect of appreciation and joy in life.