Let's get this out of the way straight away: If you are not an only child then you will not understand.
End of. It's like, if you're a man you can't experiance anti-female sexism. If you're old you cannot experience anti-youth ageism. It's just not possible. And therefore, I can try as hard as I like to explain it to you, but you will never understand quite how it feels.
It feels really crap. Quite often it feels lonely. It leaves you as an insular child who is sometimes precocious (often described as being 'comfortable in the company of adults' by those parents who have only one child), difficult to form lasting relationships with and is hideously self-absorbed. I am 16 years old and I still feel entitled to everything because I always had everything. I had all my parents time and all my parents love and all my parents attention and I have had to learn the (very) hard way that I cannot have that from everybody else. In all fairness to me, I never had horriffic tantrums - that wasn't my style - but I was bloody used to getting my own way and if I didn't get it I would sulk. I still sulk today, any of my friends will tell you. And before someone points it out, I know not every only child is like this - but a fair few are.
Secondly, it can be really fucking lonely. Sometimes it hits me even today. Yesterday, my friend was telling a story about being introduced as their 'little sister' and yet again I have to acknowledge the fact that I will never have a sister, and nor will I ever be one. I will never know what it is like to grow up with someone and to share your nightmares and your holidays and your illnesses and your arguments with them. When my parents die they will not live on in anyone but me, and I will not be comforted at the graveside by anyone who knows exactly how I feel. I will never have any relationship more than a friend or a lover. I will never be bonded to anyone by blood and DNA. I am cursed to have an absense, eternally.
I struggle with forming deep friendships. I struggled with other children. I still do feel more comfortable with adults, which makes me uncomfortable among large groups of my peers.
Again, I am an extreme case. I feel that I have actually greived the loss of flesh and blood in my time. I have cried myself to sleep over it and it will be a wedge between my parents and I - because yet again they are more people who don't understand. Although, the benifits include the fact that we have a very close relationship.
And, if I hadn't been born an only, I may not have been a writer at all. My 'let's pretend' games where I created alternate families where I had older brothers and younger sisters and pets grew into stories, laboriously recorded in little notebooks that I still have somewhere. I got used to my own company - a benifit and a drawback - and I have grown to be able to detatch myself from emotions when necessary. Clearly there are the benifits, including my parents complete and total love, which I will be ever-grateful for. But, I still wouldn't choose it.
[This is poorly written, but I am sad and I am not at my best.]