Why do some partners’ overlook the good in their partner and pinpoint the bad?

I’m not just going to point at females’ because I have witnessed guys’ doing this as well, Basically one partner is constantly complaining and bitching to their partner about something that needs fixed or needs cleaned like their arms are broken and can’t do it themselves , and completely overlooking everything else their partner has done , like they are never satisfied and always finding some sort of flaw to bitch about , and acting like their partner isn’t good enough , My ex pulled this shit on me as well , always bitching or complaining about something small and acted like I was this horrible person that didn’t see it before she did , and completely overlooked everything else I have done , pretty much another reason she is my ex. A partner that constantly complains and constantly nagging to you over little things , do yourself a favor and kick their asses to the curb where they belong , life is too short to be with someone that brings constant misery into your life. Like they walk around acting like they are your boss and expecting you to be every profession to appease them , tell them to Go fuck themselves and to get out of your life immediately
Why do some partners’ overlook the good in their partner and pinpoint the bad?
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