In a relationship or marriage, it's really easy to fight about almost everything. The point of this phrase is to remind the other person that you realize you aren't perfect and you never will be. You are trying to tell them that if they want to complain, choose the most important things to complain about and you will try to fix those problems. You are trying to tell them that it's too stressful to hear about everything that is wrong with you all the time. It makes you tired and it makes any problem difficult to fix. It is so easy to be fighting all the time about pretty much nothing
This is important in all relationships. Even if you are upset with your boss at work, it's important to choose your battles wisely instead of complaining about every little problem that happens. If you choose your battles wisely, you will win more of the important things and maybe lose on a bunch of small stuff that isn't too important
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