David was a small boy. It is impossible for a small boy to defeat a man who is over 9 feet tall and is a trained warrior. That whole scenario is enough to laugh at, as Goliath did. It wasn't the fact that David had the courage, blind faith, or stupidity (depending how you look at it) to run out on the battlefield with rocks and leather. It was the power of God. It would not have mattered if Goliath was 20 feet tall with steel skin. God is mightier. The story is not for David's glory, it was for God's. If you read about the rest of David's life, he was quite the screw up. Yet God used him over and over for His glory.
I think our biggest mistakes when confronting life's biggest challenges, is that we look to gain glory from the victory. Great life victories are for the glory of God. Because of this, I would bet that we aren't conquering like we should because of our prideful view. I know I am not conquering the way God could. So when we read that after David killed Goliath, he cut off Goliath's head and held it up for all to see, he was not showing off. He held it up proclaiming the great victory from God. Our Goliath's today may be in the form of different struggles and circumstances, but regardless, let's go into battle for the glory of God and lift up the heads of our Goliaths!