The overwhelming majority of people I know have at least one sibling. Every once in a while, I remember that I'm an only child, and I'm like 'oh yeah, actually that might explain a lot about me'. There are some supposed characteristics (positive and negative) of being an only child that I identify with extremely well, such as having abnormally high ambitious and feeling more comfortable in the company of adults (assuming I'm still a kid, age 21 still is in my opinion) as opposed to other kids. And other things such as having learned how to enjoy my own company from a young age, and sometimes just feeling a bit different and not really feeling like I belong to a community/part of a group, or even having any enthusiasm about it. Being the only inheriter of my parents' DNA has always given me a strange feeling of loneliness at the back of my head. That feeling never really goes away, even though I have a good, close friendships and everything. But maybe everyone gets that, as well as the rest of these traits, idk.
^ I think there's a lot of truth to this article, particularly towards the end.