My goddaughter is an only child. Her mother would send her over all the time so she could experience siblings. We had a conversation in regards to this and we found that only children have a challenge with patience.
My best friend ( her mother) had only one child to cater to. So every wish was taken care of, simply put, only children do not have to wait their turn. Some might not think that that is a big deal, but in life it really is an important skill.
My goddaughter was more pampered because she had 3 adults to do her bidding. Because of that she acquired demanding habits that when she came to my house, I could not entertain. Because with her included I had 3 kids to take care of. An example, we had hot dogs for lunch. I've got three very hungry kids, all under 5. She sent her hot dog back to me because her condiments were touching. I told her to get over it and eat it. I didn't have time to cater to her specific issue. That might sound mean, but she did eat it, and she never had an issue with her condiments touching at my house again. She thanks me every day for teaching her that this life wasn't all about her and how to wait her turn.