Yeah, philosophy forum was probably better. Many bright folks over there.
Have you heard of one highly favored piece of speculation by theologians concerning the fall of Satan? Namely, that God showed all the angels, of whom Lucifer was their chief, that in the plan of salvation a mere limited human would be the king, God’s own son. Lucifer in his pride could not accept to worship God as human, despite it being God’s will, and irrevocably chose his way instead. He was cast down, or perhaps more accurately cast himself down.
Of course, if we accept this, we could take your question to the next step - what exactly instilled in Lucifer a sense of pride? Is that something angels are naturally prone to? I suspect we are getting beyond our limits of comprehension here. However, like with most philosophical questions, we must ask ourselves does it even matter what the answer is? If we indulge this idle curiosity/speculation too much, it can lead us astray of the real meaty lesson - to love God, reject Satan, and beware of pride.