Between three and five years, a child learns that what they think is not what you think.
It creates separation where the child learns to lie: The stuff in their mind-box is secret.
In intimate relationships, we do not want to keep secrets. So we open up all our boxes for the other to see. Or we discard the ones we cannot share.
As adults, we get to learn this: The existence of a box does not need to be a secret. The contents can stay private.