"The root of our sinfulness is the desire for our own happiness apart from God and apart from the happiness of others in God... All sin comes from a desire to be happy cut off from the glory of God and cut off from the good of others. The command of Jesus cuts to this root, exposes it, and severs it. [Matthew 22:39 Love your neighbor as yourself.]
Another name for this root of sinfulness is pride. Pride is the presumption that we can be happy without depending on God as the source of happiness and without caring if others find their happiness in God. Pride is the contaminated and corrupted passion to be happy. It is corrupted by two things: (1) the unwillingness to see God as the only fountain of true and lasting joy, and (2) the unwillingness to see other people as designed by God to share our joy in him. If you take the desire to be happy and strip away from it God, what you have left is the engine of pride. Pride is the pursuit of happiness anywhere but in the glory of God and the good of other people for God's sake. This is the root of all sin."
- John Piper, What Jesus Demands from the World