With almost all of my close friends having siblings, I have to say: being an only child kinda stinks. Sure, getting your parents' attention and having gifts all to yourself is great, but it doesn't really seem fulfilling. Some of the stuff my parents gave me was great, but because I was an only child I didn't really have anyone to play with. At least with siblings, you can do something in your household also having all that attention can hinder you long-term.
Having siblings is actually a good thing. Though I don't really know the situation behind the scenes of the relationship, it's still better than being an only child. First of all, it's great being the only child at first but in time you start becoming spoiled. I myself was spoiled back then. The fact that my parents gave me gifts and focused all their attention on me made me believe that I was entitled to everything. Now flash forward to today, and I now that I have a better understanding of my parents and their situation back then, I hated my past self.
That's why it's bad to be an only child, because you have everything in your own hands, and it's very hard to see it go away. Having a sibling at least makes us mature a lot faster. Having responsibilities like taking care of the little brother/sister allows us to manage time and just do things at a much more mature rate rather than just doing nothing and who knows, the eldest might be a role model to the younger because of how mature and responsible he/she was.
Being the youngest can also be a type of motivation. Trying to prove that you're better than the eldest is a sign of you trying to constantly improve yourself AKA growing up. Sibling rivalry is great! I've seen siblings pushing each other whether it's in the academic or sporting world to be the best. I wish I had one. With me being a competitive type of person I'm sure we would've pushed each other as far as we could.
Another reason would be the 'growing up' phase. When you have all that attention, it results to having everything coming easily for you. As great as that sounds, if you don't learn to take care of yourself like managing your time or learning how to do things by yourself in the future, then the real world will eat you up.
I did not grow up until mid-high school. All of my friends had a plan after high school like going to college out of state or looking for jobs to pay for their own personal finances, and then there was me. I was thinking of nothing but going home and playing video games. Sure, I had marching band, but other than that I haven't given any thought to planning something. This was because my parents kept taking coddling me so much that I was not prepared for the real world sooner.
Yes, having a sibling is annoying, and yes, being an only child gives you more attention. However, if you have no siblings, it's gonna be hard for you to mature and take care of responsibilities, which is why I always wish I had one.