I've been thinking about this a lot recently as the parent of an only.
IT's by choice, sort of, in our case - there's no secondary infertility is what I mean. However we do have very particular reasons why remaining with one child is a more sensible decision.
Staggered to have been quite aggresisvly questioned and judge recently by three separate people as to 'when' we are having a sibling for DD and 'why' are we not. 'She will always have someone there for her.' 'She needs a brother or a sister.' 'How can you let her be alone for the rest of her life?' 
It's interesting reading this thread because at the same time as these (astonishngly rude and upsetting, and from virtual strangers btw) comments have been cominhg my way, DH's best friend is in the midst of sheer hell with his 3 siblings as their parents become infirm and in one case terminally ill.
The poison, vitriol, toxicity of what is going on between these 4 siblings is mind-blowing. It is all about inheritance and about who is doing the lion's share or otherwise of the care.
I suspect at least 3 out of the 4 of the siblings will never speak to each other again after the parents are gone.
Yes, it's extreme, I suspect. Though I know personally of two other families to whom this exact thing has happened and it's mind-blowing how bad it can get.
I know a good deal of other friends/friends of DH's who just have no real interaction with their adult siblings now, no major drama, they just go years without seeing each other.
I do, yes, know some whose siblings are their best friends. A few. Not that many.
It's why it strikes me as odd (even ignoring the inappropriateness) that people are so SURE that you must give your child a sibling so they have a friend for life.
Obviously nobody can ever really imagine their own small children at daggers-drawn 50 years hence over inheritance. If I had a second child, I totally get that I would always assume, even cling to the hope, that they would always be pals and get along. But just because we WANT our kids to get along later in life and can't imagine that sweet little 5 and 7 years olds playing togther now would ever be calling thier lawyers to write letters about Mum's jewellery, doesn't mean it won't happen.
I always say this these days: if I could wave a magic wand, what I would want for DD when she is older, and facing life without us (not 'alone' but you know what I mean) is that she has a wonderful, caring, supportive partner to be there for her and keep her safe and loved.
I sometimes wonder whether this fervent wish - that your child has someone to hug them, be there for them when you are gone - is a driver behind child number 2 (for those who don't just want another baby because they just want another child, that is. I know plenty who feel that way, but also I know plenty who didn't feel that way but were swayed by the 'must' have a sibling logic).
But looking at my own circle, a kind and supportive partner is of WAY more importance to those I know than their siblings are. And I even count those who are best friends with their sisters. THe presence - or lack - of a lovely relationship in their lives is a bigger factor in their feeling of happiness and security than a sibling is.
Obviously as parents, you can 'control' whether or not they have SOMEONE if you 'give them' a sibling. You can't control who, if anyone, they will meet in 30 years time to set up a home with.
But it amazes me that people still think a sibling is a guarantee of a friend for life. It can be. But when it isn't, and ime it often isn't, boy can it go horribly wrong.
THat's my two-pence worth on the siblings issue!
Still upset that virtual strangers opined on my hard-thought family choices to me in such aggressive terms, tbh.
I've