I never even meet most of my family in person. I know I have 4 first cousins from my fathers side, and I only met one I person. However my mother isolated herself from her family, so I only ever met my second cousin Chad from her side.
It's also pretty lonely being a first child, you kinda learn to live with being isolated in your own world, however you will crave the attention of others. In order to get the human interaction I so dearly desired and rarely ever received from my family, I resorted to spending half my life on the computer interacting with other users. Also, I know that sometimes if your an only kid your parents put all the pressure on you to do good and live up to all their expectations (luckily this never happened to me, as I put too much pressure on myself. Instead my parents had to calm me down so I wouldn't freak out over something minor like a bad grade).
Though I do admit that only child does have it's perks. Like for example, parents will get you almost anything you ask for (within reasons of course). My parents are currently planning on getting costume parts for me to play Ruby Rose from RWBY, nearly 2 days after I mentioned I wanted to be her next Halloween. Another one is that they can easily manage all your health and school records.
[Sorry for this being so long, I'm very in depth when describing emotions XD]