I was researching something (verifying that I accurately remembered that Seraphim are the highest choir of angels) when I found this attributed to Saint Thomas Aquinas regarding the nature of the name "Seraphim":
"The name "Seraphim" does not come from charity only, but from the excess of charity, expressed by the word ardor or fire. Hence Dionysius (Coel. Hier. vii) expounds the name "Seraphim" according to the properties of fire, containing an excess of heat. Now in fire we may consider three things.
"First, the movement which is upwards and continuous. This signifies that they are borne inflexibly towards God.
"Secondly, the active force which is "heat," which is not found in fire simply, but exists with a certain sharpness, as being of most penetrating action, and reaching even to the smallest things, and as it were, with superabundant fervor; whereby is signified the action of these angels, exercised powerfully upon those who are subject to them, rousing them to a like fervor, and cleansing them wholly by their heat.
"Thirdly we consider in fire the quality of clarity, or brightness; which signifies that these angels have in themselves an inextinguishable light, and that they also perfectly enlighten others."
Since the Seraphim would not be their own source of "Illuminating Fire," it's logical that this property is derived not just merely from God per se but due to their relative spiritual proximity to God. In other words, because they are the highest angels they exhibit more of the qualities of God: in this case the piercing, penetrating, brilliant, radiating fire. This would correspond with an idea I've had that the Fire of God is everywhere in eternity and that it's the disposition of one's soul which determines whether this penetrating and illuminating fire is glorification or damnation. It sure gives a concreteness to the notion that the souls in hell damn themselves more than God sending them there of His volition.
It would also suggest that the ranks of hell are "geographically inverted" from the way we think of it: that the higher (less severe) levels of hell are mystically farther from God such that the intensity of God's Fire affects them less than those who are closer/at the deeper levels of hell... and that attaining certain Sacramental characters (Baptism, Confirmation, Holy Orders) puts a leash on damned souls, preventing them from fleeing further from the Divine Fire than souls lacking those characters.
Another consideration: the highest of angels, being closest to the source of the illuminating Divine Fire, in communicating this perfection to the other angels... was the Light Bearer (Lucifer). This would also explain why the Sacred and Immaculate Hearts are always pictured as being on fire, and it makes me wonder if martyrdom by fire is the most glorious way to exit this mortal coil.
Lots of food for contemplation here and it makes me wonder if this Truth is why every human being is inexplicably drawn to stare at a campfire... because at a certain level we know we're seeing a glimpse of the most fundamental Truth there is.