Here's the relevant transcript from the Yale Milton Course. I guess Satan suffered from a sense of unfairness/injustice from the meritocratic breakdown of the Son's exultation (or maybe from loss of status). Divine Justice being unknowable was Milton's message, I believe.
To account for Satan’s fall, Milton relies – he makes all this up and it’s magnificent. He’s relying here on the text of the second Psalm, a chapter of the Bible that he had himself translated into English in 1653. Now the second Psalm imagines the Messiah speaking. This is that verse: “[T]he Lord to me hath said, Thou art my Son; I have begotten thee this day.” No one has ever known what to do with that passage from the second Psalm. The idea that the Messiah could actually remember the day that God had begotten him had for centuries and millennia provided biblical commentators with a paradox that really bordered on the absurd. What could God have meant when he said, “I have begotten thee this day”?
Milton worries and worries this problem of this begetting. He does this throughout Paradise Lost and he does it in a number of ways. He transforms this mysterious declaration that he has lifted from the second Psalm into – he’s turned it into what is essentially the originary event of the entire poem. As you can see from the chronology of the poem that Fowler gives us, the exaltation of Milton’s Son of God (and Fowler unfortunately calls him Christ) doesn’t really function as a Christ. Milton would never in a million years, certainly, in Paradise Lost call this fellow Christ. He doesn’t call him Jesus either because the Son of God isn’t Jesus yet. He’s preexistent but he’s not a Christ, I think, in part because Milton doesn’t want us to confuse or to construe this Son of God with anything that we’ve learned from any of our Sunday school classes or any exposure to traditional, orthodox, mainstream Christian Protestant or Catholic thinking.
This is the event – you’ll remember how important first events are in Paradise Lost – this is the event that seems to have happened first. What occurred in heaven before this moment, the poem gives us no definitive clue of, although Satan knows there is something important that happened before this moment, and he makes an important conjecture about it prior to this. (We don’t have time to look at this today, but you may want to study for yourself Satan’s own theory of a first Creation, and that’s the creation of the angels.) So God the Father assembles all of the angels. This is page 316 in the Hughes, Book Five, line 600. God delivers at line 600 a pronouncement that is by any standards absolutely shocking. Satan, like the other angels, was a son of God but, unlike the other Son of God – or the other sons of God – Satan had been one of the first archangels in heaven. The narrator suggests (this is coming from the narrator) that Satan may have been the first archangel in heaven, first both in God’s favor and also in his general preeminence over the other angels because God liked him most, liked him best, and also because he was simply better than all of the other angels; and Milton continues his meditation on the multiple meanings here of “first.”
Now I’m going to read this passage to you, and you tell me if you think Satan might have just a teensy bit of a reason to be miffed at God’s exaltation of another one of his sons, or what we have to assume is another one of his sons. This is God at line 600:
Hear all ye Angels, Progeny of Light,
Thrones, Dominations, Princedoms, Vertues, Powers,
Hear my Decree, which unrevok’t shall stand.
This day I have begot whom I declare
My only Son, and on this holy Hill
Him have anointed, whom ye now behold
At my right hand; your Head I him appoint;
And by my Self have sworn to him shall bow
All knees in Heav’n, and shall confess him Lord:
Under his great Vice-gerent Reign abide
United as one individual Soul
For ever happy.
What’s the tone of voice? I’m [laughs] going to interrupt myself for a moment. How are we [laughs] to imagine God pronouncing that word “happy” after what he’s just told us?
Under his great Vice-gerent Reign abide
United as one individual Soule
For ever happy: him who disobeys
Mee disobeyes, breaks union, and that day
Cast out from God and blessed vision, falls
Into utter darkness.