Gregory the Great
Catena Aurea · Matthäusevangelium 18
Take heed that ye despise not one of these little ones; for I say unto you, That in heaven their angels do always behold the face of my Father which is in heaven. For the Son of man is come to save that which was lost. How think ye? if a man have an hundred sheep, and one of them be gone astray, doth he not leave the ninety and nine, and goeth into the mountains, and seeketh that which is gone astray? And if so be that he find it, verily I say unto you, he rejoiceth more of that sheep, than of the ninety and nine which went not astray. Even so it is not the will of your Father which is in heaven, that one of these little ones should perish.
(ubi sup.) We must consider whence it is that the Lord declares that He has joy rather over the converted sinners, than over the righteous that stand. Because these last are often slothful and slack to practise the greater good works, as being very secure within themselves, for that they have committed none of the heavier sins. While on the other hand those who have their wicked deeds to remember, do often through the compunction of sorrow glow with the more heat in their love of God, and when they think how they have strayed from Him, they replace their former losses by gains following. So the general in a battle loves best that soldier who turns in his flight and courageously presses the enemy, than him who never turned his back, yet never did any valorous deed. Yet there be some righteous over whom is joy so great, that no penitent can be preferred before them, those, who though not conscious to themselves of sins, yet reject things lawful, and humble themselves in all things. How great is the joy when the righteous mourns, and humbles himself, if there be joy when the unrighteous condemns himself wherein he has done amiss?