Defensive behavior can take many forms. It can be anger, the shutting down of discussion, using one's authority to intimidate staff, communicating by body language that one is unhappy that a topic has been broached, marginalizing people who disagree with them. All of us are guilty of it from time to time but healthy individuals work hard to live non-defensively with a nothing to prove, nothing to lose attitude.
Why is defensiveness so unhealthy?
First, it comes from our own dishealth or poor EQ. Not once in the gospels do you see Jesus defensive no matter what the pharisees goaded Him with - and they tried hard! He listened to them, He was rarely reactive and when He was it was never without a purpose (driving the moneylenders from the temple). He often responded to the goading of the Pharisees with a question. On the other hand the Pharisees were continually displaying defensive attitudes.
Defensiveness is part of our sinful nature and the need to prove something or to be right or to keep our pride. It is really self righteousness. Non-defensiveness, on the other hand is not worried about any of those things. It comes from a healthy place that is self-defined but not needing to prove anything. For healthy people, life is a journey toward not needing to prove things to others. We are OK with their opinions because we are OK with ourselves and our positions.
Second, defensiveness hurts relationships. For those of us who are married, think of how damaging some of our conflict with our spouses has been because neither of us was willing to admit fault or give on our position! Defensiveness pushes others away while non-defensiveness invites them into relationship.
Third, defensiveness hurts us. Usually we are defensive over something because we don't want to be challenged, and know that there is some truth in what is being said. Why would we be defensive if there is not some truth involved? If we listened and asked some questions it might be that we would learn something. We have everything to gain by listening and nothing to lose.
Again, I would look to the example of Jesus in the Gospels and the Fruit of His Spirit and ask whether defensiveness should be part of our lives.