Jamieson, Fausset and Brown on Luke 18:21

21. All these, &c.--"what lack I yet?" adds Matthew (Mt 19:20). Ah! this gives us a glimpse of his heart. Doubtless he was perfectly sincere; but something within whispered to him that his keeping of the commandments was too easy a way of getting to heaven. He felt something beyond this to be necessary; after keeping all the commandments he was at a loss to know what that could be; and he came to Jesus just upon that point. "Then," says Mark (Mr 10:21), "Jesus beholding him loved him," or "looked lovingly upon him." His sincerity, frankness, and nearness to the kingdom of God, in themselves most winning qualities, won our Lord's regard even though he turned his back upon Him--a lesson to those who can see nothing lovable save in the regenerate.