1. You must be lonely.
2. You're spoiled.
3. You get loads of attention.
4. You're selfish.
I couldn't think of any more remarks but I do find these insulting.
While I am an only child, I wasn't lonely. I'm an introvert by nature and never craved brothers or sisters. I still don't. I have my family and I'm happy with that.
Far from being spoiled, I had to work for anything I wanted. It wasn't that my parents were trying to instill a work ethic, it was that I didn't want to ask them for anything. We had a less -than-idyllic relationship and I preferred not to interact with them if it could be helped.
As for attention, my parents were too caught up in their own issues to bother with me. My mother made it very clear to me at an early age that she no time for me other than to feed and clothe me. My father was an alcoholic who spent most of his free time elsewhere. In short, I was ignored and left to my own devices.
Selfish? I still don't understand that. I've known many people with siblings who were selfish. I have no problem sharing my time and a sympathetic ear to anyone in need.