When you are feeling defensive, that’s a sign that you’re dealing with a big/complicated problem. You are feeling defensive because you don’t want to feel the pain of having your certainties uprooted.
Unfortunately, defensiveness is a vicious self-reinforcing feedback loop: you act defensive, so you become less effective at learning from and tackling the problem underlying your defensiveness, so the problem becomes worse, so you become even more defensive about the problem, because tackling it would be even more painful.
When you see defensiveness popping up as an emotion, you should strive to let the defensiveness inform you and help you notice a gap in your understanding of the world–you should not let the defensiveness drive you and close you down to learning (because emotions aren’t always correct).