He was upset at the reaction of the army of God!
"What will be done for the man who kills this Philistine and removes this disgrace from Israel? Who is this uncircumcised Philistine that he should defy the armies of the living God?"
His older brother ridiculed him.
"I know how conceited you are and how wicked your heart is; you came down only to watch the battle."
When Saul heard about David, he sent for him.
"David said to Saul, "Let no one lose heart on account of this Philistine; your servant will go and fight him."