Most children with siblings are pretty sure that they would have been better off as only children. They don't buy into the whole "lonely only" propaganda. I don't actually know very many only children, but I imagine there's a lot of pressure.
I claim that the first is born of ignorance and all subsequent children are born because having the first one killed (or at least numbed) parental brain cells. My daughter and I have another explanation, though. We believe that our parents looked at the first child and said "We can do better than THAT," and had us. They stopped because they had reached perfection. Guess where we are in birth order?
I don't know why parents expect children to be happy about getting a new baby brother or sister. I read something once that really made it all very clear to me. Imagine if your spouse sat down with you one day and said "Honey, I love you so much that I want to have another one just like you. We're going to bring this new member of our family home to live and play with us, all the time. Won't that be wonderful?"
My son thought that was about the funniest thing he'd heard all day. Of course, he's the older child. What does he know?