people learn how to act when they are interacting with people, and while watching people interact with each other, and other times when they are thinking about those interactions.
you might ask, 'well then doesn't that mean that siblings will all act the same?' no because no two people have the exact same ideas about how to act.
everybody is in their own unique situation, not shared by anyone else on the planet, now or ever. when they interact with people, and when they watch other people interact, they interpret the meaning of the interactions -- and when we interpret we are using our ideas to do it, and since no two person's ideas are identical, no two people will interpret everything the same way.
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