St. Augustine
“Thus the true cause of the blessedness of the good angels is found to be this, that they cleave to Him who supremely is. And if we ask the cause of the misery of the bad, it occurs to us, and not unreasonably, that they are miserable because they have forsaken Him who supremely is, and have turned to themselves who have no such essence. And this vice, what else is it called than pride? For ‘pride is the beginning of sin’ (Eccles. 10:13).”
Source: St. Augustine, City of God