William Burkitt on Luke 18:21

This assertion might be very true, according to the Pharisee's sense and interpretation of the law, which condemned only the gross outward act, not the inward lusts and motions of the heart. An external obedience to the letter of the law this man might have possibly performed; this made him think well of himself, and conclude the goodness of his own condition.

Learn hence, how prone men are to think the best of themselves, and to have too high an opinion of their own goodness and righteousness before God; this is very natural, but dangerous and fatal.