Saul appraised David’s chances at success based on outward appearance and worldly wisdom. He couldn’t see David’s heart. For if he could, he would’ve seen the heart of a lion, one who was after God’s own heart. Besides, it pleases God to take the small to overcome the great. In this, God receives greater praise. God cares more for your availability, than your ability. David made himself available. And God did the rest.