Out of all people I would consider to be a friend and someone I spend time with I only have one friend who is an only child.
I love my friend but she does always seem to want her own way. Such as I organised a meal and cinema night, when I got there she had changed the film we were seeing and the restaurant.
We had a night in on New Year and I said lets play this game, and she replied "oh but I wanted to play this game instead, we can play that other one later." Which of course didn't happen.
A few friends have seen her do this and just remark that it's probably because she is an only child and used to getting her own way which does make sense.
However like I said I only know one person who is an only child so I can't generalise.