I am an 'only' and agree with many of shimmy's points. I did not however really long for a brother or sister when young, it's only in retrospect I feel it might've been a good thing for me - lightened me up a bit, perhaps as a kid.
I was a quiet,introspective sort of child (although I know a little girl now who is the complete opposite to me in personality as an only child..) I still have no problem with my own company and rather crave it after a while. I feel now at 40 it might be nice to have a sibling - no-one else really understands the 'dynamics' of a family like those that are part of it, so friends cannot always fill that gap. It's not something that eats away at me though. I had a lovely childhood and that's what counts totally. I know many people have brothers and sisters whom they barely see, so maybe I have a somewhat rosy tint on what it's like.
Also agree with the 'parents getting older' comment - although mine have always been fit and well and had me when they were young, they are now running into some health problems which worry me. Again though, I know of examples where despite there being siblings, problems to do with the welfare of the parents often fall to just one of the children, so who's to say that a brother or sister would be around to help or show an interest? Many don't it seems to me.
My parents were fun to be with because they were young themselves, but I can see it can become a bit of a 'triangular' relationship - almost like you are part of the adults and have to grow up to catch them up and join in with them. Every success I had never went un-noticed - I had their full attention, but likewise, any failure on my part was rather magnified. Nothing to offset it against, which could become tiresome.
Only children can often feel quite 'dutiful' to their parents and don't wish to disappoint - I know I am very like that even now which annoys me a bit about myself.
I wanted more than one child for some of the above reasons. I got more than I bargained for though, and feel quite bemused sometimes observing my three together. The noise, negotiations and endless shifting hierarchy of it all is quite beyond me at times - I am learning it now with them.