As the ointment on the head, which descended to the beard, to Aaron’s beard, which descended to the fringe of his garment.What was Aaron? A priest. Who is a priest, except that one Priest, who entered into the Holy of Holies? Who is that priest, save Him, who was at once Victim and Priest? save Him who when he found nothing clean in the world to offer, offered Himself? The ointment is on his head, because Christ is one whole with the Church, but the ointment comes from the head. Our Head is Christ crucified and buried; He rose again, and ascended into heaven; and the Holy Spirit came from the head. Whither? To the beard. The beard signifies the courageous; the beard distinguishes the grown men, the earnest, the active, the vigorous. So that when we describe such, we say, he is a bearded man. Thus that ointment descended first upon the Apostles, descended upon those who bore the first assaults of the world, and therefore the Holy Spirit descended on them. For they who first began to dwell together in unity, suffered persecution, but because the ointment descended to the beard, they suffered, but were not conquered…. (in Ps. 132)
St Thomas Aquinas:
It is becoming for those who apply themselves to the Divine ministry to be shaven, and to be tonsured in the form of a crown by reason of the shape. Because a crown is the sign of royalty; and of perfection, since it is circular; and those who are appointed to the Divine service acquire a royal dignity and ought to be perfect in virtue. It is also becoming to them as it involves the hair being taken away: both from the higher part of the head by the tonsure, lest their mind be hindered by temporal occupations from contemplating Divine things, and from the lower part by shaving, lest their senses be entangled in temporal things (Suppl. 40, 1).
I’m not sure how to overcome this aporia, unless we think that Hanon in 1 Sam. 10 had the right idea by shaving off one half of the beards of David’s servants. However, Cornelius a Lapide says that this episode represents the Jews plucking off hairs from Christ’s beard during the Passion, or the devil stripping religious men of their courage, so that does not seem promising. I suppose we have to say that per se it is better for a man, especially a Christian, to possess a beard, at least in this life, but per accidens, e.g. because of a particular need to signify something else, it may become better to be shaved (and tonsured). I say ‘in this life’ because Aelianus tells me that the earliest depictions of our Lord present Him as risen and beardless, in token of eternal youth, and suggests that this may be the ‘other form’ which hindered people from recognising Him after the resurrection.
I presume that the historical reason why priests in the west have generally been shaven is Romanitas. Eastern rite priests of course often have beards, and I have a theory that the East-West division here is a providential counterbalance to the characteristics of their respective liturgies. That is, the Eastern liturgies put before us in particular the glory of the resurrection, so it is fitting that their priests be bearded, to preserve some suggestion of the trials and labours of this life as well, lest we float off into unreality. On the other hand, the Roman rite is more stark and sacrificial, so perhaps there is a danger that adding beards as well might make it too much for some people to take. I hope these are not irreverent thoughts.