It's hard to be your own quality control. It's hard to make sure you behave as well as you can, if all you have in your checking toolbox is the same algorithm that made you do the thing you just did. It's hard to improve if you constantly believe you did the best you could in the situation.
Sure, we all do the best we can in each situation, given circumstances, upbringing, traits and the like. But surely, if we knew what we know now, we would've behaved differently in many situations. Or would we? I pose that we oftentimes wouldn't. We oftentimes behave worse than we know we could, knowingly, or at the very least knowingly unknowingly, we let things through and ignore that we did. An acidic comment to our partners, a rabbit hole or a moment as Karen when we could've been polite.
What's the antidote to this? Well, I can totally see two Karens ganging up on a poor waitress regarding a bill. Group think. So just adding people doesn't seem to prevent us from not being able to check ourselves.
We need a different perspective. It'll be painful, oftentimes, but it will allow us to grow. It'll allow us to know, just that little bit more that our perspectives have been checked from two angles. It'll allow us to see things with different eyes.
Now how do you acheive this? It's simple. Choose a partner who is more unlike you than like you. Choose a partner who likes to talk things through. Choose a partner where you do have differences in fundamental values.