I honestly think that people who moan and bitch about anyone having an only child, and who judge and berate, and say shit like 'she will be lonely' and 'she will be 'spoilt' etc, regret their choice of having 2 or more. They have multiple children, and they regret it because things didn't turn out as they had hoped, they struggle with them, and they want you to feel as wretched as they do.
Not saying people who have more than one child are all like this - at ALL! But I do believe the ones who are bitchy and vitriolic about 'only' children, (and also the parents of only children,) are bitter and jealous. Especially as a parent of an only child, will often have a fantastic bond with their child, that parents of multiple children don't have.
I have had conversations with people with 2 kids, who said that although they love both their kids, the dynamics of the relationship with the first born changed a lot when the second one came along. In addition, they don't always get on, they sometimes fight a lot as kids, and they often don't have great relationships as adults.
I also know a woman who had a daughter when she was 28, and had the most amazing relationship with her; they did loads together, and she and her daughter and husband were like the 3 musketeers. A very happy solid unit.
Then when the daughter was 9, they decided to try for another baby, and had a son when the daughter was 10. (The woman was 38 now.)
Long story short, the son had multiple health issues including learning difficulties and quite severe autism. The amazing relationship with her daughter was gone, as the son had to always be put first now.
It also split her and her husband up after nearly 2 decades together. The daughter went to uni 2 years ago, the son is now 10, and my friend is now alone - a single mother with a son with lots of problems. She is 48, she looks 60, and she had to give up her career and social life to be his carer. (Not sure why it was HER who had to give up her life, because the boys's father's life hasn't changed one iota.) 
Bollocks to them @Sarahh2014. Tell them to mind their own business. How many children you have is YOUR BUSINESS. If you are happy with one, and your life is great with one, then for the love of God, stick with one!!!