The radio speaker this morning related how David would not hurt King Saul when the opportunity for vengeance arose although Saul had tried to kill David on several occasions. David was always conscious of the fact that Saul was anointed by God. Although he himself had been told that he would succeed him as king, he refused to avenge himself because he respected God's authority that was invested in Saul. The Bible declares David to be a man after God's own heart.
The speaker summed up the message by saying that God does not select men based on their intellect or their abilities. David was just a shepherd. God looks at the heart of men and on that basis he picks.