I was an only until I was 12, then both of my parents (separately) had a second child, so I suddenly had two sisters as I entered my teens.
I am also the parent of an only child (aged 3). Part of the reason we chose to have one child is that I can honestly tell her she's my favourite girl.
I do worry that she might miss having a sibling (as adults, I'm really close to one of my sisters, and I'd miss that relationship if I didn't have her), I also worry that when my husband and I are older (and die) she will find herself alone, and the unthinkable, unmentionable worry, what if we lose her? As much as no sibling could compensate, it would give us a reason to keep going without her.
But over all, we have a brilliant little family, we have just enough money to be comfortable, just enough time to give her our very best, just enough energy and patience to surround her with love. I never really dreamed of having 'kids' per se, but I am absolutely thrilled to have HER. I know I'd love any child, but I might not LIKE another as much as I like her.
She attends group-daycare, she is sociable, confident, and popular (in ways I have never been). She is doted upon by us and the wider family, but at the same time we have expectations of her behaviour and attitude, so although she's probably a little spoiled materially, we are very aware of not allowing her to become entitled.
I'm 95% sure that having another child isn't for us - I'm almost counting down the days to menopause when the decision is taken out of my hands.