My husband and I were together for 9 years before we married, and we KNEW we wanted 2 kids, for all the right reasons. A few months after our daughter was born, we both (guiltily) admitted that maybe 1 would be plenty. 3 years later, we don't even feel a twinge when we see our friends welcoming their second children to their families. There are many many good reasons to have an only child, and many many good reasons to have more. I definitely agree it's a decision that should wait a few years. But since you asked, here are all the reasons we decided that our daughter completes our family:
She was born a "high-need" child. My own just-to-myself, never-share-this-with-anyone term for her was "intensely unhappy". This was true for 8 months. Yep. 8 months of intense unhappiness. Just imagine waking up every day to an intensely unhappy housemate. One that demanded, demanded, DEMANDED attention, holding, distracting, food, clean diapers (24+ a day for the first week! 12+ a day at ONE YEAR!!!). She screamed in pain every time she peed. She cried when I made eye contact (too stimulating). She could nurse-sleep for 4 hours straight, but could NEVER nap unless in my arms. I held her during naps until she was almost 1, then I had to give up. I put up with even more night time screaming as a result. She still nursed 3 to 7 times each night at 18 months. I was a zombie.
Fast forward 3 years. We survived. We're stronger. We're never doing it again. We are just starting to get our own lives back, each of us having one night a week to get out and do something with friends. She is big enough to do some really fun stuff now, and we're having a lot of quality family time as a result. Yes, we work hard to give her social opportunities. Yes, she struggles with taking turns and sharing. Yes, she still cries when I let Grandma babysit. But we'll get there. And this IS the right decision for us.
So, if the child described here resembles your child, you may be looking for a light at the end of a tunnel. But if your child gave you more joy than headaches for the first year of life, maybe you should think twice about the only-child idea. In the end, it's your family, and your choice.