No One Lose Heart

1 Samuel 17:32
David said to Saul, "Let no one lose heart on account of this Philistine; your servant will go and fight him."

This comes only a few verses after the last heart. After gathering all the information he could on the Goliath dilemma, David had an answer, and he himself was it.
He uses heart to mean courage, spirit, energy. With a supernatural maturity, a fatherly concern, he perceives that courage, spirit and energy are draining from the Israelite army as the standoff continues. They've talked themselves into terror in the intimidating presence of the big Philistine.
Maybe his shepherd training taught him this. He was used to being the brains, courage, and heart for a bunch of fearful, stupid sheep, and his instincts told him the Israeli army was about to stampede off a cliff.
Yet he didn't trust in himself to reverse the trend. He trusted in God alone, and it was on God's behalf that he went out to meet Goliath.
Just as Saul failed his first test, David passed his first one.