I will apologize up front for not being an only child and still posting anyway.
But, I wanted to address something that I haven't seen mentioned here yet. I was very sick with our first and wasn't sure I could handle going through it a 2nd time, and our first was a boy and a handful, so the idea of a 2nd to take care of was a little stress inducing.
But, we never wanted our child to grow up alone. So we have 2. And I must say that it affected ME (in a good way) in ways I wouldn't have expected. I am a better parent now. Why? Because having more than one forces you to keep things in proper perspective for your children. It is very easy to succumb to today's societal pressures and let your child rule your world. But that is not what is best for the child. They need to be taught how the world does NOT revolve around them, which is completely counter to what they instinctively believe. They learn to share and care for others at a very early age.. so that THOSE concepts become normal, not a forced effort later on. And there will never in this world, EVER, be another person who will have the same shared experiences as your child, except for a sibling. When they are 25 and getting married, no other person in this world (except for a sibling) will be able to share the memories they have. No, mom and dad, their memories are not the same as yours... you are the authority.. they are the children... VERY different perspectives and memories of the same things. I am not really close close with my siblings, but we were close at different times growing up, and the closeness waxes and wanes with the stages of our lives. I can even recall long distance phone calls from college to each other (at different colleges). Now that my parents are aging, I am glad that I have my brothers (and their wives) to share in the process.
Fwiw, my son (3 yrs older than his sister, now 11 yrs and 8 yrs) are BEST FRIENDS and get along famously. There is the occasional typical squabble, but by and large, they play together and love each other in an awesome way. There is nothing sweeter than the sound of the two of them whispering in the back seat and laughing together, when I don't even know what they are talking about. Or when one is with me shopping and gets some sort of treat and asks me for another to take home for their sibling.