When you say offensive, this means that you are attacking someone. When you are offensive, this means that you are the one who is initiating an active attitude. When you say that you are defensive, this means that you are receiving all of the actions of someone who is being offensive towards you.
There is a big chance that someone who is being defensive may try to block the actions of someone who is being offensive, but there are also times when the defensive person is meant to fight back, especially if it is a life or death situation.
When you are defensive, there is also a possibility that you are trying to challenge all of the criticism that is being thrown at you.