He couldn't understand why no one answered the Philistine's challenge. David found courage in his conviction of what he knew was right. For the Christian, this conviction can likewise be obtained through the study, understanding and application of Scripture. To this idea even the Roman Senator Cicero agrees. Conviction of right and wrong is necessary if courage is to be developed.
Conquests strengthen courage.
David was experienced at facing fierce opposition and threats to his life. As was said earlier, David had acted bravely when he didn't feel like being brave, facing the lion and the bear. Now, when Goliath threatened, his courage was strong enough to meet the test. One of God's purposes for the trials of life is to strengthen our courage. God lets us endure threats and danger to demonstrate to us that He will be present with us in our struggles.