I was an only, but had always wanted someone else around to play with and when my parents divorced I became even more keen for a sibling I could talk to about everything that was happening. I think I would have been very jealous of any children that my mum and stepfather had, mostly because he and I didn't get on, so I was then glad to have no sibling coming on the scene.
Now my parents are older, and i often wish i had a brother or sister around as both parents are on their own again now and although i love them and it is lovely to have them around a lot there are times when i'd like a little more space, or when one of them in particular gets very down mentally i find that hard to handle, it would be great to have someone to chat to about that (though DH is excellent at listening) and to take up some of that pressure. When my parents die I'll have no one to remember anything from my childhood with. I'll be well and truly alone, and having seen both parents go through funerals of their own parents with siblings to help I can see how hard it will be to do that by myself. Not the practical side, I'm very organised and I know I'll be fine with that bit, but the emotional side. Both parents sat with their siblings on the death of their parents and talked about childhood memories and laughed and joked together about it all.
To me, even if they don't always have a close everyday relationship, siblings have a bond that is irreplaceable by any other relationship, much like a parent child relationship.
Having said all that when dd was born I struggled with parenthood for the first couple of years, which shocked me massively, and I swore no more kids. Then she grew, I found it easier and she stated asking for a sibling...a LOT. Eventually, with some trepidation on my part we did conceive again and just before dd's 5th birthday her brother arrived.
I found having no 2 MUCH easier at the baby stage, and although now he's nearly 4 they DO argue and bicker and wind one another up, there are also manifold reasons why I am delighted that we made the decision that we did. Dd adores her brother, and although he is less demonstrative with her I know he adores her too. Seeing them playing together, her reading to him, them holding hands as they walk on the beach, hearing their giggling when they are up to mischief together, all make me melt. I would never have had that with just dd, and I think she would have missed out on one of the most rewarding and challenging relationships she'll probably ever have. Having ds around has taught her so much, and having her makes his life far richer and more fun than if it has just been DH and I.
It's better for me too. My tolerance and selflessness has definitely increased, and i believe i'm a nicer person for it (not saying this is true of everyone, or that people with anonlyarenotthe same, just talking about my own case here).
Overall yes it is more work in some ways, but it's far less in others. I don't regret having two for a single second, I just wish we'd had ds sooner and had had time to fit ina third!