I am a single child. When I was a kid, I was a bit selfish. I didn't want my parents to divide their attention and love between my sibling and me. Hence, I was always against a sibling. So, as a single child. I did all the love and attention and every resource my parents could afford to give me. Now that is a pro. Another good thing is that I don't have to fight with my siblings, because there is none.
But again, the con is that since there is no sibling, I don't know who I should fight with.. Or whom to show love to, apart from my parents. Most of the times when I feel lonely, I would love to share it with someone of my age group in the house. But I have none. So, that could be a problem of being a single child.