As the only child, the first thing a person says to you when you tell them is, "oh you must be spoiled" or "you are so lucky," but actually it's the worst. Being the only child is a blessing and it is very delightful, but it also shaped me into this weird, awkward, anxiety-filled being that I am today.
Not growing up with siblings and always being around my parents made me more mature than others. I never really knew how to socialize with others and I tended to isolate myself, which caused me to not have any friends. I was outcasted as a weirdo and I was selfish. Since it was just me in the house I always had all the attention and had a hard to accepting and finding myself getting jealous of others. I hated sharing and when people would come over for playdates l always gave them the old/dirty toys.
During my childhood, I was very sheltered which didn't help my situation at all. My parents wouldn't let me have friends over, I couldn't go over to my friends, and I couldn't even cross the street. My parents were like my best friends, I couldn't be away from my parents for too long without having separation anxiety and I could never stay the night anywhere without crying and begging to go home.
Having siblings isn't vital in my opinion, but it does help develop proper socials and the feeling of not having all the attention. Being the only child is literally the worst for your social skills, but it isn't all that awful. Since I spent so much time with myself I learned so much about my self and became more self-aware. So if you are an only child or plan on only having one kid: find the time or find time to help your kid become more self-aware, but to always keep other people's feelings and thoughts in mind.