Lol, what a shock. You're a parent of an only, talking for them.
This is unheard territory, a parent of an only child, declaring how great their child has it.
Take a seat next to my parents...
According to you, only children are desperate for siblings
Some. This is for the fourth time now. There is some real "hard of reading" on this thread. And as for your child not telling you, only children tend to be constantly seeking the validation and approval of their parents. When you're all they've got, they don't always tell you they aren't happy, because they can't afford to lose the only immediate family dynamic they have.
are unhappy through life
Aaaaand you've made that up too. But isn't it interesting how my actual words of "longing for a sibling" you've translated as being unhappy through life. Very interesting. I'm happy. There is a massive void in my life where a sibling should be. I wonder what your child will say as an adult. I'm sure you've already got that answered.
and their parents pretend they are happy
Yes, many do. Some, because they can't admit they might not have made the best choice for their child. And some, like my parents think that all the stuff they can throw at just one child, makes that child happier than a sibling.
I'll do it for a fifth time, as it seems necessary, not everyone will have the same experience.
Oh, and I just have to laugh at you thinking I've got a miserable life. How sad. I have a far better life than you could probably ever hope for. Incredibly privileged, all the things that people work, wish and strive for, way beyond the norms, done. Maybe that will help it sink in how much a sibling can mean to a child, when I would rather have none of it, but a brother or sister.