Gregory the Great — as excerpted in the Catena Aurea on Luke 15:1-7

“(ubi sup.) But he allows there is more joy in heaven over the converted sinner, than over the just who remain stedfast; for the latter for the most part, not feeling themselves oppressed by the weight of their sins, stand indeed in the way of righteousness, but still do not anxiously sigh after the heavenly country, frequently being slow to perform good works, from their confidence in themselves that they have committed no grievous sins. But, on the other hand, sometimes those who remember certain iniquities that they have committed, being pricked to the heart, from their very grief grow inflamed towards the love of God; and because they consider they have wandered from God, make up for their former losses by the succeeding gains. Greater then is the joy in heaven, just as the leader in battle loves that soldier more who having turned from flight, bravely pursues the enemy, than him who never turned his back and never did a brave act. So the husbandman rather loves that land which after bearing thorns yields abundant fruit, than that which never had thorns, and never gave him a plentiful crop. But in the mean time we must be aware that there are very many just men in whose life there is so much joy, that no penitence of sinners however great can in any way be preferred to them. Whence we may gather what great joy it causes to God when the just man humbly mourns, if it produces joy in heaven when the unrighteous by his repentance condemns the evil that he has done.”