I’m an only child and it didn't consciously bother me at all as a child. I was very happy in my own company, was able to entertain myself very well, don't ever remember seriously wanting a sibling other than out of vague curiosity.
However once I hit adulthood I realised that there had been issues in my childhood that resulted directly from being an only, which I didn't fully comprehend at the time. For example finding it very difficult to make friends at school or, later, university. This has improved over time - as a result of me working quite hard on it - and I’m more or less over it now, but I'm certainly not someone who craves the company of others (so lockdown not a big deal for me tbh).
The other thing was the kind of intensity of the relationship I had with my parents, which I really didn't enjoy, from my teens onwards. We were very close when I was a child and then it really deteriorated after I went to uni, with lots of expectations (not career related - more the emotional stuff). I guess I had the stereotypical 'delayed teenage rebellion' when I was in my 20s. They are both dead now and while I miss them very much, there's a part of me that's relieved if I’m honest, and not just about the fact that they both died relatively young (early 70s) so that I never had to deal with the whole dementia and care home life stage for them. It's also more about the pressure being off, if that's not an awful thing to say.
My DS is also an Only (not my plan at all, but I had him very late so that was that) and I made very sure that he would be around other kids as much as possible from the earliest age. He likes having friends way more than I ever did, but it's drifting more the other way as he gets a bit older. I’m not sure I’m doing so well on the 'not having expectations' front though and I worry about that a bit!
He occasionally says he wishes he had a brother, but I just tell him they'd probably fight. He has a close friendship with a neighbour's DS who's just a bit younger and they're almost like brothers - and they DO fight, so he gets it.
What I feel more guilty about tbh is that I haven't been able to provide him with aunts, uncles and cousins (or, as it turns out, grandparents). On my side of the family there's just me, some step-relations and some distant cousins, none his age, and all miles away, so basically no-one. He's got some cousins on his dad's side whom he gets on with but again they're a bit older. That lack of family age peers is my biggest regret because I had that growing up too (I can remember being always literally the only child in the room at family gatherings) and I hated it, and it's SO not what I would have chosen for a child of mine.