Unless its a special needs boarding school or the DC has no siblings and is desperately lonely at home how is it NOT selfish?
How is it NOT judgy to assume that's true of everyone who boards that that if they do so for reasons other than the two you've given is 'selfish'?
I boarded as a teen as my father worked in an industry that crashed in the early 80s, it was work abroad or be unemployed and the place did not have a high school, so the company arranged for my sister and I to board.
We loved it. We are a loving, happy family.
My son has SEN and he wanted to board. It's been great.
A lot of pupils board as their parents work abroad, are in the forces, want to go, specialist school, SEN, the education system in their home country is unsuitable, they want to become fluent in another language, etc etc.
But I'm finding that it's always easier for the hard-of-thinking to just throw out 'selfish' to anything that doesn't fit in their tiny boxes. It's tedious in the extreme.