whats the most IMPORTANT thing in a relationship ????

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there are a lot of different types of relations that can be characterised , but my question is just a simple that whatever the relation be whether it is PARENTS/CHILD...FRIENDS....GIRLFRND/BOYFRND....HUSBAND/WIFE....BUSINESS PARTNERS...ETC. what is the most important ingredient in these relationships that make them last longer or make them stronger . Try to answer with a SINGLE reason or maybe TWO even .