This is a really interesting thread for me. Where I grew up boarding school wasn't a thing at all - we read about it in Enid Blyton but actual boarding schools didn't exist where I lived and the only context in which I heard about ones further away was one of punishment - my best friend's mother threatened to send her to the one up the country if she didn't behave, but it wasn't a real threat - sending your child away was seen in my circle as being very extreme, no matter what the reason was.
It was only when I moved to the UK that I met people who had actually been to boarding school and who considered it 'normal' - it totally baffled me, I couldn't get my head around parents voluntarily giving up their children when they didn't have to.
Through work I became friends with a wonderful woman who boarded from the age of 9. Like a lot of people on this thread she doesn't talk about it much but I think because I was so fascinated with it/didn't consider it normal she found it easier to talk to me in a way - she didn't have to explain why she feels the way she does about it. I got the sense that when she mentioned it to other non-boarders they considered her a bit of a show off because there is such a class element involved in the whole thing - at one point a mutual friend implied that she was spoiled for complaining about such an 'opportunity' - having other people envy something that caused you such pain must be so isolating.
When we first met she described herself in almost exactly the same words as other posters have used here - resilient, independent, self-contained, private. That's not what I saw at all. I saw such a lot of fear - it was like she lived her life on the edge of a precipice that she believed she could fall over at any point. I know it's not a great analogy but I likened it to a stray animal who knows what human love and affection is but who now fears humans - they crave attention but can never let their guard down to actually get it, they constantly move away, they're jumpy and overreact to everything and it's exhausting for everyone around them. She just didn't believe that anyone would have her back. And why would she? The main people in the world who should have had her back left her when she was 9, voluntarily. She had to learn that even 'unconditional' love isn't dependable, that you can be separated from it without much real cause (her parents sent her to boarding 'for her own good')
I agree with what other posters have said that there was a layer of socialisation missing. I think people who grow up in a family learn through long, hard and often difficult experience in the teenage years that their family will be there for them no matter how 'wrong' or 'bad' or 'ugly' they are -so much of teenagerhood is about 'rebelling,' which is really a process of testing the boundaries of the love of those close to you to discover your own worth. If you don't live with those people, then the opportunities to do that are practically nil - because the time together is so short lived, it's all editing and nicey nicey - who wants to go home for only two hours and spend that time having a fight? Anyway, there's noting to fight about - there's no narrative as such, your lives are essentially separate. And if the people you interact with most are staff, rather than family, you're not going to test them, because there is nothing to test - they don't love you and you have to accept that. God, it is so sad. That 'resilience' comes across to me as a sort of scar tissue, hard and uncomfortable, that my friend bears from the ongoing hurt of rejection. She is just used to it now and has never had experience of the healthy version, where you push and are accepted over and over until you believe it.
The people who staff the boarding school may be lovely, kind, caring, etc etc (in fact, many at my friend's boarding school sounded like wonderful people) but the fact always remains that they are not your mother/father/brother/sister and that is significant. The idea that it isn't a problem absolutely boggles me tbh - I don't understand how some people can miss so completely the significance of a parent - is that because their own parents were distant/abusive? Parents are special. Your mother isn't just some kind woman, she's your mother, there is only one of her. Someone can act as a poor substitute, but what's the point in a substitute if the real one is right there??? Why would a child choose a kind stranger who doesn't love them over their actual mother?
Now that I'm a parent I understand the whole thing even less. My son is close to the age that my friend was when she went boarding and I had to tell her how much it upset me to think of her at that age, crying silently in her bed, knowing nobody would come to her, never having a mum in the other room who would come running at the first sign of distress. It is such a loss and so unnecessary - that's what gets me. Her mother was alive and healthy and could have been there, cuddling her. But she wasn't. For no reason. My friend just nodded when I told her - she does appreciate being told these things (she feels she needs to be 'recalibrated') but she thinks if she really lets herself feel the impact of what I say she'll fall apart. She won't. But she will have to accept a lot of really hard things, like the fact that her suffering wasn't really necessary. I think she worries that if she accepts that she'll hate her parents.
I understand that boarding can be helpful in awful situations - abuse, death of a parent, etc. - it is filling in a gap when the best option is no longer there. But why does a parent choose it? Children are young for such a short time, why as a parent would you just decide not to be part of it? Why deny yourself the chance to live with your own baby?