Getting defensive is hardwired in humans and is universal. Driven by our amygdala and our "fight or flight" response, it's a natural reaction when we are feeling "attacked". When we get defensive, we only focus on the negative aspects of the conversation and are unable to recognize anything positive. We may naturally go on the defensive if we already have a negative relationship with someone in which trust has been lost.
Everyone can think of a time when they felt themselves getting defensive when someone was talking to them. You may not yet have the self-awareness to recognize it at the moment, but you recognize it later when rethinking the conversation.
As an exercise in emotional intelligence, it's important to be able to recognize at the moment when one is getting defensive, and equally important if we are entering into a situation in which we know we will get defensive. Having the self-awareness to recognize when you are getting defensive, will allow you to practice self-management and in turn, relationship management. When we are feeling defensive, we are very much in our emotional mind and need to practice pausing to give ourselves time to move through our rational and wise minds.