Ambrose of Milan — as excerpted in the Catena Aurea on Luke 15:1-7

“Now the angels, inasmuch as they are intelligent beings, do not unreasonably rejoice at the redemption of men, as it follows, I say unto you, that likewise joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons who need no repentance. Let this serve as an incentive to goodness, for a man to believe that his conversion will be pleasing to the assembled angels, whose favour he ought to court, or whose displeasure to fear.”