Certainly his brothers thought he was a "cocky little git." Consider, he comes out to the camp with some food for his brothers, and hears Goliath ranting down in the valley, asks what is going on, and is incensed that the army of Israel is too chicken to answer the challenge! He says, "Who is this uncircumcised Philistine, that he should defy the armies of the living God?" -- and his eldest brother basically says, "Shut up and go tend the sheep, youngster!" and David replies, "What did I say? What did I say?"
David was not well liked by his brothers, I think. Some months previously the prophet Samuel had come to his home, looking for the man who would be King over Israel after Saul. Samuel had looked at all of Jesse's sons, and God said to him "Not this one." Finally Samuel asked Jesse if he had any more sons, and Jesse said, "Well, my youngest is out tending the sheep." When David was brought in from the field, Samuel anointed him with oil. This anointing was an honor in their culture, and it probably rankled his brothers that David received this honor and they did not (in their culture, the oldest brother was normally held in the highest esteem, the youngest in lightest esteem). So it is not surprising that his eldest brother snarled at him when he came out to where the army was camped.
I think, though, that what his brothers took as a cocky attitude, was really an attitude of faith that he would prevail. God had brought his people through many trials, and He would bring them through this one. They just needed someone to stand up and act on his faith in God. Since no one else was doing that, David decided that he would.
LoboHunter, I also typically carry five stones around with me these days. But unlike David, I would need all five in order to hit something! Oh, well, David had probably been slinging for seven or eight times as long as I have been.
I am not sure whence came the idea that the stones were flat. All the translations I have looked at, read "five smooth stones from the brook." That would be nice, rounded rocks.