My mum had fibroids, and several miscarriages. As a small child, an only child, I felt very wanted.
I think being an only is swings and roundabouts. I had every adult attention, was probably precocious and more articulate than many of my peers as I was more with my parents than other children. I felt very loved and secure, had many toys, books, experiences with my parents. When I failed the 11+ they were able to afford an expensive education that they would have struggled to provide for a sibling. I had friends, but was lonely, especially during secondary school holidays, as my independent school was at a distance from my home town, and my friends with it.
Growing up, I felt very strongly that I wanted my family to consist of more than one child- not purely because of childhood loneliness, but also from a feeling of adolescent suffocation, when to my eyes my mother, in particular, was loathe to let me grow up.
On top of that, as others have commented, dealing with your parents as they age and fail is heart breaking- I think I would have appreciated a sibling to share, off-load, and vent with. Having said that, DH does not feel that his sister is of support in this. Luck of the draw, I suppose.
There are no guarantees in life. But I hope my pigeon pair, who as youngish adults are not particularly close, will always look out for one another, care however loosely for each other, be able to chat to each other about “ times when..” Be someone else in this world who has a head start on “getting” you.
I notice that I share many similarities with a friend who is another only. We are quite certain of our correctness in most things, a little ruthless/selfish in our determination to do as we think fit, both of us outgoing and friendly, but also both harbouring secret worries that we may have read social situations wrongly, both of us a little over sensitive to perceived criticisms. She too decided on two children, pretty much for the same reasons as me.
We have been very lucky to have the choice.