Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
And when the words were heard which David spake, they rehearsed them before Saul: and he sent for him.
And David said to Saul, Let no man's heart fail because of him; thy servant will go and fight with this Philistine.
And Saul said to David, Thou art not able to go against this Philistine to fight with him: for thou art but a youth, and he a man of war from his youth.
And David said unto Saul, Thy servant kept his father's sheep, and there came a lion, and a bear, and took a lamb out of the flock:
And I went out after him, and smote him, and delivered it out of his mouth: and when he arose against me, I caught him by his beard, and smote him, and slew him. Thy servant slew both the lion and the bear: and this uncircumcised Philistine shall be as one of them, seeing he hath defied the armies of the living God.
David said moreover, The LORD that delivered me out of the paw of the lion, and out of the paw of the bear, he will deliver me out of the hand of this Philistine.
And David said to Saul, Let no man's heart fail because of him; thy servant will go and fight with this Philistine.
And David said unto Saul, Thy servant kept his father's sheep, and there came a lion, and a bear, and took a lamb out of the flock:
And I went out after him, and smote him, and delivered it out of his mouth: and when he arose against me, I caught him by his beard, and smote him, and slew him. Thy servant slew both the lion and the bear: and this uncircumcised Philistine shall be as one of them, seeing he hath defied the armies of the living God.
David said moreover, The LORD that delivered me out of the paw of the lion, and out of the paw of the bear, he will deliver me out of the hand of this Philistine.
And Saul said unto David, Go, and the LORD be with
thee.
And Saul armed David with his armor, and he put an helmet of brass upon his head; also he armed him with a coat of mail.
And David girded his sword upon his armor, and he assayed to go; for he had not proved it. And David said unto Saul, I cannot go with these; for I have not proved them. And David put them off him.
And David girded his sword upon his armor, and he assayed to go; for he had not proved it. And David said unto Saul, I cannot go with these; for I have not proved them. And David put them off him.
Vs.31-39
Thou art but a youth — Supposed to be about twenty-two or twenty-three years of age.
*David, as he had answered his brother's passion with meekness, so he answered Saul's fear with faith, and gives a reason of the hope which was in him that he should conquer the Philistine, to the 
satisfaction of Saul.
But he argues from experience; though he was but a youth, and never in
the wars, yet perhaps he had done as much as the killing of Goliath came
to, for he had had, by divine assistance, spirit enough to encounter
and strength enough to subdue a lion once and another time a bear that
robbed him of his lambs.
To these he compares this uncircumcised Philistine, looks upon him to
be as much a ravenous beast as either of them, and therefore doubts not
but to deal as easily with him.
If you are growing cold and careless,
if you are getting sleepy and
dull,
rebuke your soul,
and use your past experience as a whip wherewith
to flog yourself into energy.
Let it never be said that he who woke
himself up to fight a lion now falls asleep in the presence of a
Philistine.
David remembered that by confidence in God his energetic
fighting gained the victory— the lion was killed, and the bear was
killed too. And cannot you remember, brethren, what victories God gave
you?
He could not see a lamb in distress but he would venture his life to
rescue it. This temper made him fit to be a king, to whom the lives of
subjects should be dear and their blood precious, and fit to be a type of Christ, the good Shepherd, who gathers the lambs in his arms and carries them in his bosom.
