I am on only child.
If you are not an only child I suspect your default idea of me now is that I was spoilt, that I'm not very sociable and that I don't know how to share.
If you are an only, or a parent of one, you will feel differently I imagine. Like the child at a school who speaks with an accent on the first day the only and the only child's parents tend to over compensate for the above reactions (the child at school normally sounds local by day 3). So it was drummed into me as a child that I was spoilt (I was of course, most children are, but I was always told if I knew I was spoilt then I hopefully wouldn't be too insufferable); I went to an activity almost every day after school when it was unusual, to make sure I wasn't unsociable or awkward (that particular stereotype is especially silly as clearly the only child attends far more grown up parties and goes to far more events alone and therefore learns much earlier to bound in and say hello) and I come from a mostly happy family (anyone who says their family is perfect is fibbing in my opinion).
Still I find only children and sibling relationships fascinating. The love hate of siblings is something I don't understand at all, though I wish I could. Also, despite all the efforts above I do really enjoy, even crave, time on my own. Though saying that there are times when I feel like I am quite alone in a wider sense and that is sad, but I can sit in the house by myself or get lunch for on quite happily really. I do of course know only children who are the total opposite of me. I do also think as time goes on I am a bit more unusual than other people, rather quirky, but I might have been like that anyway.
The onlyness, which is how I think of it, is perhaps the reason I'm not good at making relationships last, I expect something fairytale even now. Onlys seem to be very work focused too and I'm not sure how healthy that is. I'm a very true and loyal friend and I imagine that's because I don't have brothers and sisters to worry about or care for and perhaps I project those feelings onto friends.
Still I don't know anything else and I embrace being an only- and one day I might write a guide book to prove we aren't weirdos!