God’s and Samuel’s criteria for who should rule are not necessarily the same.
Samuel was initially impressed with Eliab, Jesse’s eldest son, because of his impressive height and physical prowess. God’s warning to The judge who anointed the first two kings of Israel More that a person’s stature is not a reliable measure of an individual’s qualifications is, on the surface, reminiscent of Samuel’s earlier experience with Saul, who was also remarkable for his height (9:2; 10:23) but also rejected by the Lord (15:23).
A theological principle, however, is also involved in this memorable verse. Samuel’s limited perspective arose out of his inability to see beyond outward appearances. Since Eliab (and The first king of Israel More!) had apparently been blessed with good looks, it was natural to assume that God’s blessing would extend to leadership. But God states very clearly that there is much more involved: “The LORD looks on [better: “into”] the heart.” In the thought world of the Old Testament, the “heart” represents what we would call “the mind”–the seat of the will, the intellect, and decision making–attributes that Samuel could not “see,” but which were of the utmost importance and divinely discernible. God will be the one to choose Israel’s king, and it will be on the basis of God’s criteria.