"The root of our sinfulness is the desire for our own happiness apart from God and apart from the happiness of others in God. I mean that to be read carefully. Let me say it again: The root of our sinfulness is the desire to be happy apart from God and apart from whether others find their eternal happiness 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 [to love our neighbor] cuts to this root, exposes it, and severs it. 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 our 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 as the fountain of your happiness and strip from it people as the ones you hope will share your happiness in 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, from "What Jesus Demands from the World", commenting on Jesus' command to love our neighbors as ourselves, pages 256-257.