Adam Clarke on Mark 10:21

Then Jesus, beholding him - Looking earnestly, εμβλεψας, or affectionately upon him, loved him, because of his youth, his earnestness, and his sincerity.

One thing thou lackest - What was that? A heart disengaged from the world, and a complete renunciation of it and its concerns, that he might become a proper and successful laborer in the Lord's vineyard. See Mat 19:21. To say that it was something else he lacked, when Christ explains here his own meaning, is to be wise above what is written.