Advantage:
The entire world is yours! Literally! You can do what you want and get everything you want without thinking.
You can invest a lot of time exploring yourself.
You can achieve anything and almost anything with extreme determination.
To be so protected and pampered that you are the center of attention of everyone in your family.
You will be blessed with something called "out of the box" and 99% will work.
Your thinking would be structured that way! You will be very creative, not even many can think of your imagination and creativity.
You want to be unique and dominate everything. You will not just understand these things.
If you have to do something, you will take it with the utmost professionalism, learn from scratch, and try to master it.
You don't save anything by name! Instinct! You have a strong instinct that alerts you early to anything wrong with your area.
A fixed decision-maker. It takes too long to conclude, but if you do, no one can blame you because you would have made all the permutations and combinations that others would not have even thought of when making a decision.
The strongest support! You will support your loved ones no matter what, even if they commit murder, and the will and determination to show your support will threaten anyone who resists you and your loved ones!
Disadvantage:
Very possessive! You will be very possessive. You can even assassinate those who try to touch yours, either for a specific cause or for your people.
It's really hard for you to tolerate your people being close to other people, and it will multiply your love for them.
Solitude! They often think that you are being denied all attention, and even if you have done all the work perfectly, you will feel depressed for no reason at the end of the day, even if you have everything in life.
Sharing is a big deal! I have personally experienced this. When someone asks me for something, I buy a new piece of what they want and give it to them instead of giving them what I used.
I develop an emotional bond even with the little ones I have, and it has become a habit for me. Pamper! They ask their loved ones to pamper them when they feel bad. And you make sure to limit the number of people who should be close to you because you only let those who love you and understand your stupidity stay with you to pamper you and make you feel good.
Uncertainty! Even when my dad or someone teases me for supporting others, I sometimes feel as unsure as a newborn baby would feel if someone hugged their mom! If someone surrenders to you and breaks your trust and if they do so in front of the world, you will not show your insecurity out loud, but you will make sure to close all doors for this person and eliminate them definitely out of your life.
Ambivert! You seem introverted in the eyes of the world, but you want to express each of your thoughts with your loved ones with so much emotion! What sometimes hurts when nobody listens to you!
Disadvantages:
The advantage of being a single child to parents is that you get all the attention, love and resources of the parents. YOu enjoys center of attraction all the time, no one to share parents with and so on.
On the other hand, being a single child brings number of disadvantages to the child. He/she has no sibling to play and fight with that affects social development at large. There are certain attributes of life which are learned when you have companion such as sharing, perspective taking, inter-personal skills, empathy and so on. Moreover, after parents, it is your sibling which gives you an ultimate sense if belongingness irrespective of harsh life circumstances. So, single child miss that.