My 8 year old son is an only child ( at the moment anyway! ) and he went through a phase recently always asking about having a brother or sister. What was funny was that he seemed to assume that his sibling would come out old enough to play AND leave his stuff alone! Not like his baby cousin who cries, takes up all her mama's attention and has stinky diapers! He has always been a very happy, well- adjusted child. If anything, he was precocious, because he interacted with adults a great deal, but being an only child has never interfered with his making lots of friends and being outgoing in groups, etc.
At the moment, he is sort of getting the best of both worlds, because we moved near a family with FOUR kids, all his age, two boys and two girls and he became best friends with one of the boys. Their mother loves having my son over ( says she doesn't even notice one more) and it is really exposing him to having to deal with a lot of other kids and their needs. He loves playing with them and being part of the gang- but then he is MORE appreciative when he comes home and can have all the popcorn in the bowl and no one has moved his stuff from where he left it or whatever!
I had one brother growing up and it was great, but I think only kids turn out fine too! Don't worry about people's comments - I remember when I was going through my painful divorce and very few people knew about it. Strangers and acquaintances would tell me how great my son was- and then add the comment about 'when's his little sister coming along? You don't want to wait too long!" etc. That was the LAST thing I wanted to hear. Just ignore it- peopel don't mean to be rude, but sometimes they are!