Particular friendships are necessary for seculars (but not for monastics). St. Francis de Sales wrote in ch. 19 of
Introduction to the Devout Life (https://isidore.co/calibre#panel=book_details&book_id=6312):
QuoteFor since in a well-regulated monastery the common aim of all tends to true devotion, there is no need to form particular friendships there, for fear lest, making a particular aim of that which is the common aim, they pass from particular friendships to partialities; but as for those who live in the world, and who embrace true virtue, it is necessary for them to form holy and sacred friendships with one another; for by this means they encourage one another, help one another, and lead one another on to good.
Ecclesiasticus 6:7 (http://drbo.org/cgi-bin/d?b=drl&bk=26&ch=6&l=7-#x): "If thou wouldst get a friend, try him before thou takest him, and do not credit him easily."
St. Alphonsus di Liguori,
The True Spouse of Jesus Christ (https://isidore.co/calibre/#panel=book_details&book_id=7683), ch. 10 "Detachment from Relatives and Other People" (ref:836.1), ยง2 ("Detachment from seculars") lists (ref:836.37-42) St. Bonaventure's criteria (
De Profectu Rel. l. 2, c. 27):
- It is not pure when it leads to long and useless discourses; and when conversations are very long they are always useless.
- When each delights in looking at each other and in praising each other.
- When either excuses the other's defects.
- When they manifest certain little jealousies.
- When either feels unhappy at being separated from the other.
Also, a good litmus test "of the carnality of spiritual affection" is "if the passion of love (
passio amoris) precedes the affection of the will (
dilectionem voluntatis)" (
De veritate q. 26 a. 7 (https://isidore.co/aquinas/QDdeVer26.htm#7) "Does a passion accompanying a meritorious act detract from its merit?" ad 7).
I imagine the Angelic Doctor and Seraphic Doctor fulfilled the opposite of the above criteria in their mutual friendship, each having entered the beatific vision a few months apart.