While I don’t have firsthand knowledge of being an only child, I do have many friends who are. For whatever reason, their nuclear family includes only one child.
As I was growing up, there were many times I wished to be the only child, to have all my parents’ attention. I especially wished it when my younger sister and I disagreed and argued. All of us with siblings have, at one time or another, probably wished to have been the only child. Of course, that was in my younger and less mature days, and now I realize how much I value having a sibling.