16:7 his appearance … height of his stature. Samuel needed to be reminded that God’s anointed was not chosen because of physical attributes. This was initially a difficult concept for Samuel as he was accustomed to a king whose only positive attributes were physical. the Lord looks at the heart. The Hebrew concept of “heart” embodies emotions, will, intellect, and desires. The life of the man will reflect his heart (cf. Mt 12:34, 35).
MacArthur Study Bible
16:7 man looks on the outward appearance, but the Lord looks on the heart. Outward appearance cannot predict whether someone will faithfully obey the Lord, for a person’s actions flow from his heart (cf. 2 Chron. 16:9; Ps. 51:10; Prov. 4:23; Mark 7:21–23; Luke 6:45; 1 Thess. 2:4). The “heart” in Scripture refers to a person’s inward moral and spiritual life, including the emotions, will, and reason.
ESV Study Bible