John Chrysostom

Then since Jesus mentioned those out of the law, he saith, "All these things have I kept from my youth up." And neither at this did he stop, but again asks, "What lack I yet?" which itself again was a sign of his very earnest desire.

Homily on the Gospel of Matthew 63 ↗

Theophylact of Ohrid

The young man saith unto Him, All these things have I kept from my youth up: what lack I yet? Some accuse him of boasting and arrogance. For how could he have achieved love for neighbor if he were rich? For no one who loves his neighbor as himself is wealthier than his neighbor. Others understand it thus: suppose, he says, that I have kept all these things - what do I still lack?

Commentary on Matthew ↗

Jerome

The young man said to Him: All these things I have kept from my youth, what do I still lack? Jesus said to him: The young man lies. For if he had fulfilled the commandments: You shall love your neighbor as yourself, how then, upon hearing: Go, sell what you have and give to the poor, did he go away sad, for he had many possessions?

Commentary on Matthew ↗

Thomas Aquinas

The young man said to him: all these have I kept from my youth. After the Lord delivered the doctrine of common salvation, here he delivers the doctrine of perfection. And first he delivers the doctrine; secondly the necessity of this doctrine; thirdly the reward of observance. The second at and Jesus said to his disciples; the third at Peter answering etc. And first the occasion for delivering the doctrine is presented; secondly the promulgation; thirdly the effect. …

Commentary on Matthew ↗