I was the youngest of 6. There were things about that I loved, things I hated. I was socially inept, lonely, had few friends, couldn't share, was greedy...in short I had many of the traits that are attributed to an only child. I loved having brothers and sisters, though, and love it in real life although I only get on with half of them (which sounds awful, but I have a couple of very toxic sisters).
As an adult, I've learned how to become socially gregarious, when necessary. I have lots of friends, am known as the joker of the pack. I have very little store in material things and am generous (I think). I lack a little confidence in some areas, but that's cos my mom hated children and I was bought up being very, very aware of my flaws!
I wanted lots of children, but it wasn't to be. I've had 6 pregnancies, one child. I love her beyond measure. She goes through stages of wanting siblings, but is currently off the idea. I've had teachers who've said, 'Huffyjunior isn't a typical only child, is she?' which is a bit insulting, but I know what she meant.
DD is loving, generous, socially confident, happy. She has lots of friends. She loves holidays, shares well, gets on with others. She struggles with adults, but she's only 10. She loves her life. I'm hoping by the time we're old, she's bogged off to Australia. I don't want her to be 'burdened' by us. But by the time that comes into being, hopefully she'll be married. And my DH was my support as my parents got old, got sick and died. Hopefully she'll find a partner to share life with and do likewise.
Being an only is different for each child, just like being in a large family is different for each child. You make the best choice you can (if you're lucky enough to have a choice) and you get on with it.