John Chrysostom

Or else; I shall not hesitate to call this ruler covetous, for with this Christ reproaches him, but I say not that he was a tempter.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

As the Lord promised the covetous ruler a treasure in heaven, he nevertheless did not heed, for he was a slave to his treasures, and therefore "became sorrowful" upon hearing the Lord urge him to part with his possessions, since he desired eternal life precisely so that with his great abundance of wealth he might also live forever. The ruler's sorrow shows that he was a well-intentioned man, and not a cunning villain. For none of the Pharisees ever became sorrowful; rather, they grew hardened. …

Commentary on Luke ↗

Bede

Hearing this, he was sorrowful, for he was very rich. This is the sadness of the world that works death. These deceptive riches are like thorns that choke the Lord's seed.

On the Gospel of Luke ↗

Bonaventure

Having heard these things, he was saddened, etc. After he showed poverty to be commendable by reason of its perfection in merit, here he shows it to be commendable by reason of the imperfection in its contrary, which is indeed the tenacious clinging to riches. Concerning which four things are introduced by the Evangelist, namely the sign, danger, obstacle, and remedy of human tenacity, which is the gift of divine grace. …

Commentary on Luke, Chapter 18 ↗