The Sin
Pride is considered the worst of the seven deadly sins. It is the sin that bothered Jesus the most, and which caused him to severly rebuke the Scribes and Pharisees and teachers of the Law in his day. He taught that God opposes the proud, and sides with the humble.
Proud people think of themselves "more highly than they ought" in view of the fact that they are no less sinners than those around them. Pride is the denial of sinfulness. As the Bible writer James put it "If we say we have no sin, we deceive ourselves and the truth is not in us...".
Pride relies on the illusion of superiority. It is costly to maintain this illusion, especially in terms of human relationships. Pride separates us from God and from the forgiveness He wishes to give us in order to bring us back into a healthy relationship with Him. When we are proud we deny God permission to change us so that we might think and behave like Jesus. Only those who are willing to change and grow "into the likeness of Christ" are useful in God's Kingdom.
Pride also separates us from the people around us who sense that they are being looked down upon. Only the most emotionally and spiritually secure people can stand to be around someone who is filled with self pride.
Finally, pride separates us from the truth about ourselves. How can we ever grow or mature as human beings if we are unwilling to look at ourselves honestly, and to take the fearless moral inventory that humility demands?