Our brains are wired for survival. But our world has changed faster than our brains can keep up with, so we have a lot of survival impulses that no longer fit our modern lifestyle and world. Many of these survival impulses are embedded in the primitive part of the brain that is largely ruled by unconscious reactions to stimulus from the environment. 

Our brains get more "lit up" as it were, from negative stimuli than from positive stimuli. The amygdala, simply put, is the part of the brain that processes fear; but it also processes positive input. Studies show that it is more sensitive to negative input because we process negative experiences (ie, feel them) more intensely. When we have a more intense experience, we pay more attention to it. One can see, from the way our brains are wired, that we give a lot more attention to and react more to negative experiences. And it makes sense that in our past, when we had a lot of dangers in our environment, our survival would depend on us being very attentive to negative stimuli.

These days we are largely safe from threats from the natural world, yet our sensitive brains are on the lookout for things to be afraid of. So, in our relationships, we are prone to noticing the negative and dismissing the positive. Does this sound familiar? Your partner doesn't notice the nice things you are working on, but still notices your faults? Or, your sensitivity to the things your partner hasn't changed trumps the things that they have changed? 

We can override this negativity bias by consciously paying attention to the positive. Actively appreciating your partner for something they are doing well by following up your appreciation with an action (a hug, a kiss, a nice smile and words of affirmation) will give the positive more energy and override the negative bias. This will give your brain and your heart a more positive view of your partner, and they of you. Practice this. Be active in changing your brain and your relationship.