"Not everything that offends us injures us; but we are driven to raving by our whims of pleasure, so that whatever does not respond as we will evokes our anger."
-Seneca's Moral Epistle XLVII
The most pervasive and dangerous anger is also the most childish and full of pride. St. Benedict of Nursia alluded to the reality that all anger is rooted in sinful pride in his Rule, teaching that humility is the proper poultice for rage. Seneca also points to this truth.
A man does not only feel anger when he perceives himself harmed at the hands of another man; he more commonly becomes angry at Lady Fortune, God, or the universe. Why? Men are quite convinced that the entire world should serve their whims. The stoplight turns red at an inopportune moment and a man takes offense. Who has wronged him? He is angry at reality itself for not bending to his will as though he were the transcendent deity.
Here is the true sin of pride at the root of anger: part of man's desire to be "like god" is the conviction that reality itself should bend to his creative will. When it does not, he perceives himself to have been wronged! How absurd to pridefully imagine that Lady Fortune, so indiscriminate in her attacks on all flesh, should step down from her throne and wait on him as a maidservant or as her unique favorite among the sons of men!
Translated from Seneca's paganism into Christian terms: we are childishly angry at the world so often because we pridefully expect (nay, demand) that God Himself should step down from His throne and allow us to command all of creation according to our whims.
For the Stoic, humility leads to the greatest achievement of the human being: life in accord with Nature. For the Christian, humility alone leads to the recognition of man's true potential: the potential to live as human creatures and leave off our prideful assault on Heaven.
How does a Christian realize such humility in this life? What tools have been given us to battle our own pride and its subsequent anger? I will point to only one here: The Second Petition of the Lord's Prayer - "Thy will be done on earth as it is in heaven." When you find yourself struggling with this pervasive and childish anger that Seneca identifies, quietly pray: "Thy will be done." In this way we can humbly remember that "whatever does not respond to our will," was never obliged to respond to our will. Only one Will commands all things, and it is not our own.
"How you have fallen from heaven, O morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations! You said in your heart, 'I will ascend to heaven; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of the sacred mountain.'"
-Book of the Prophet Isaiah (14:12-13, ESV)
Seneca's writings are available from Harvard University Press' Loeb Classical Library: http://www.hup.harvard.edu/loeb/