I was effectively brought up as an only, as my half sister is 15 years older than me and chose to live with my GPs when my somewhat martyrish DM married my narcissistic DF. I don't blame her.
I was a lonely child, I didn't have a huge number of friends (other than imaginary ones!) - I spent so much time alone, in my own little world, I was terrible at mixing and even relating to other kids who'd grown up with similar aged siblings. I envied them massively. I was crap at rough and tumble games, as I'd been wrapped in cotton wool by parents who hated me getting dirty etc. I also bore the brunt of my DM placing too much responsibility on me for her emotions. My DPs constantly reminded me how 'lucky' I was to have their undivided attention, time and finances, but honestly that was no compensation. I'd much rather have had fewer things, and a sibling close in age to share them with.
I now have 2 DCs, both boys, who are 3 years apart. I love their close bond and how they play together, and how they'll always have each other (fingers crossed). I always knew that if I had kids, I'd want at least 2, because for me the loneliness was absolutely punishing and it really set me back growing up.
I know that my experience isn't unique, and some only children loved being an only, but personally I hated it.