Agree with those who have pleaded for a little bit of tact on this. Your personal childhood experience is just that: personal to you. We also need to remember that the grass is often greener on the other side. Some people are lucky to have had a wonderful sibling relationship, either as children or as adults or hopefully both. But many of us who had one or more siblings did not have that magical experience and grew up wishing they had been an only child. And those who grew up without siblings will always wonder if they would have been happier if their parents had had at least one more child.
Nobody has a crystal ball to see into the future. If you have another child, your kids might be each other's whole world and have an amazing relationship. Or they might fight like cat and dog throughout their childhood and become like strangers as adults. The way in which you approach parenting can influence this somewhat, I'm sure, but you can't control the outcome completely.
Have another child if YOU AND YOUR PARTNER want another child. Do not do it as some sort of 'service' to your existing child.
And to those saying you have to have more than one so your kid isn't all alone in dealing with grief when you die, that is (in my opinion) a shitty reason to have more than one child. Are you assuming that they won't find a loving partner who will support them at this difficult time? That they won't have their own child/children to provide comfort? That they won't even have any really close friends to support them?
I grew up with a brother, two years younger, and I hated him. Still can't stand him now and we're in our 40s. Obviously that makes me think I would have preferred being an only child, but I can never know for sure. Maybe it would have been worse. Who knows. What I do know is that when our dad died, having a brother didn't help me at all. What got me through was the love of my husband, daughter, in-laws, and best friend.