Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
Now Saul, and they, and all the men of Israel, were in the valley of Elah, fighting with the Philistines.
And David rose up early in the morning, and left the sheep with a
keeper, and took, and went, as Jesse had commanded him; and he came to
the trench, as the host was going forth to the fight, 
and shouted for
the battle. For Israel and the Philistines had put the battle in array, army against army.
And David left his carriage in the hand of the keeper of the carriage, and ran into the army, and came and saluted his brethren. And as he talked with them, behold, there came up the champion, the
Philistine of Gath, Goliath by name, out of the armies of the
Philistines, and spake according to the same words: and David heard them. And all the men of Israel, when they saw the man, fled from him, and were sore afraid. And the men of Israel said, Have ye seen this man that is come up? surely to defy Israel is he come up: and it shall be, that
the man who killeth him, the king will enrich him with great riches,
and will give him his daughter, and make his father's house free in
Israel.
Vs.19-25
David's dutiful obedience to his father's command. 
His prudence and care made him be up early and yet not to leave his sheep without a keeper, so faithful was he in a
few things and therefore the fitter to be made ruler over many things,
and so well had he learnt to obey before he pretended to command.
How timorous and faint-hearted the men of Israel were. Though they had,
for forty days together, been used to his haughty looks and threatening
language, and, having seen no execution done by either, might have
learned to despise both, yet, upon his approach, they fled from him and were greatly afraid.
He ran into the army, to see what was doing there, and to pay his respects to his brethren. Seest thou a man thus diligent in his business.
