I would go with we experience from birth and childhood on that we cannot really completely and openly trust others, not even parents or siblings or family.
They will, at some point, sadly enough at many points in your life, let you down, put you down, humiliate and hurt you in ways both emotional and quite often enough physically as well. That is what we learn growing up. We learn to not necessarily trust people, that is the prime lesson a person needs to learn when it comes to life.
Some people you can usually or often count on, but always count on to be sympathetic and understanding? Not in my experience.
So we learn to be more cautious in revealing intimate things about ourselves. It is the 'once bitten, twice shy' lesson of life.
Going to an even deeper sociological level, there are some societies/nations (like the US or the Netherlands) that believe that open and frank, direct discussing things is the best way to proceed in relationships or with anything. Tell it like it is without any worry about another person's feelings and all will be well.
Other societies do not not think this an optimal strategy for dealing with the complexities of life. There need to be areas in a person's life that is strictly private and inviolate, because revealing all and being totally honest will also reveal off-putting characteristics that do not benefit a relationship.
i should probably clarify that when i say "conditioning" i am not thinking of that as a conscious, purposeful or nefarious agenda or effort. i think it is largely a self perpetuating state of being and acting. It's what we know. Using myself as an example (but i think we can all insert our own examples here), i was aware early on that i am attracted to guys, but was also learned early on that that was generally considered "sick, sinful, broken, perverted, etc.." And most of that was learned/conveyed through innuendo. Sure, there were those dedicated to fighting the 'homosexual agenda,' but most of my 'conditioning' was social. Kid's using terms like "faggot" as a pejorative, or making fun of and bullying the effeminate boy. i didn't tell anyone about my predilection towards guys until i was 19 years old, but i was tortured by the reality of it, and the isolation. At 12 i started looking at books on homosexuality in the library, but at that time the vast majority of those books affirmed my conditioning that did indeed happen largely through "experience." So, i think the sources of our "conditioning" vary, are vast, and mostly happens unconsciously.
i appreciate your thoughts and the perspective you bring and share. As is likely obvious from my posts, i'm big on openness in general, but especially when it comes to intimate relationship. i have a very purposeful and detailed rationale and approach for this, a paradigm of sorts, so i tend to be passionate about it. But then, my way is not 'the way' eh? lol. Your perspective that US and Netherlands are "open and frank" gave me another pause moment. Another culture that comes to mind is Asia. Though i think it way too big of a generalization, sort of like saying "white people" or "black people" when there is tons of diversity in all those groupings... but, an Asian stereotype is on of inscrutability. As a kid, i had a similar idea about the English. Watching every season of "The Great British Baking Show" has shattered that notion for me. i cannot account for the Netherlands, but have lived in the US my whole life (with visits to other countries). i think i understand where you are coming from on a comparative basis, and i can see where US culture may be more "open and frank" when it comes to some things. my impression is that if the US didn't invent it, it certainly has fostered the practice of presenting an image. There was a Cannon camera commercial that came out several years ago: Andre Agassi playing tennis and declaring: "Image is everything." Of course, it was a play on words, Cannon marketing their cameral. But it utilized a cliche,' and subsequent practice, that is prominent in American culture, that of presenting an image (vs the real thing).
To me, this is the other side of the same coin. Society/culture 'conditions' us to not only hide our true self in fear, but to additionally create and project an 'image' to people. i think it's motivated by the same universal human need/desire: to be loved, which to me is really the need/desire to be affirmed as being valuable (that reads way more clinically than i see it). The problem i see with both sides of the coin is, if i am right in believing that the "need/desire for love/affirmation" is universal, this cultural "coin" is counterfeit or what it purchases is? It seems to me one cannot be loved/affirmed if one is unseen/unknown. Others may "love/affirm" the projected image, but the real person remains unseen, unacknowledged and unloved/un-affirmed. Many remain alone and lonely in an apparent intimate relationship, but hey, image is everything.