Defensiveness is one of the Four Horsemen of the Apocalypse, which the Gottman's identify as particularly detrimental for couples in conflict.
It can be tempting to point out your partner's flaws and to imagine yourself as a victim in your relationship. Often this is when we get critical - we attack our partner's personality or character, rather than focusing on the behaviour that is problematic.
In conflict, we often see defensiveness in response to criticism. Defensiveness can be defined as self-protection in the form of righteous indignation or innocent victimhood we use in an attempt to ward off a perceived attack. It can be very easy to lean across the fence and blame your partner, to counter with pointing out their flaws "It's not me, it's you", or to see yourself as a victim "poor me, I'm justified to retaliate because you attacked me". Here is the thing, we are all responsible for our own behaviour - in adult relationships people don't 'make people do things'. The antidote to getting defensive is to take responsibility for your role in the situation or even part of the conflict. By taking responsibility for part of the conflict you can prevent the conflict from escalating. In doing so it's helpful to use I-statements.
Extra resources:
Here is a short video by Brene Brown which talks about blaming, and how when we blame we don't take accountability.
Here are two short podcasts by the Gottman Institute on the 3 types of defensiveness and how to respond non-defensively
Finally, here are some points for self-reflection from Jessa Zimmerman about owning your part:
Disclaimer: This post is for informational and educational purposes only. It should not be taken as counselling/therapy advice or used as a substitute for such. You should always speak to your own counsellor.