What factors led to your decision to have an only child?
Having a child was a HUGE adjustment for me. I think dh would be happy to have more if he were married to someone else. But he knows living with me with ZERO sleep for 4 years (dd STILL does not sleep through the night) is not fun. Honestly, lack of sleep if the number one issue. Pregnancy was fine, labor was fine, having a child around is fine. Not sleeping has seriously messed me up. There were entire months that I could not even drive a car due to severe sleep deprivation. I have no idea how people can fathom doing that more than once. Granted dd was and is a terrible sleeper by anyone's standards. And I know the next one could be the opposite. But the key word here is "could". I am not taking that chance.
Also, we spend much of our free time doing activities that are adaptable for one child but would be difficult for more. We like to backpack, cc ski, bike ride (more than just around the block), travel, etc...... We are just starting to get back into this stuff as dd is becoming more interested. I am not sure we would ever go backpacking again is we started out with another infant.
And frankly, I feel busy enough attending to my household and child as is. I am a sahm but still find ways to keep myself completely busy each day. I would not want to go back to the infant days when cooking from scratch and having a nice garden was practically impossible.
How old is your child, and do you think that your child is happy with your family dynamic?
Dd is 4. I think she is very happy. She does say she wants a sibling and I do feel really bad when we are at the park and she is alone and bored. She really likes other kids. Since we are unschooling and I usually do not have a car, interaction with other kids is rare. So, I wish I could change that, but I do not think having another kid is going to solve that problem seeing as dd would be 5 before we could even produce another one

Also, do you think you will ever reconsider adding a sibling at some point?
Not biologically. Dh is planning on getting a vasectomy soon and until then we are very very careful. We have discussed adopting an older child down the road, but I suspect that it is unlikely as our house size and lifestyle would likely not make us good candidates.
What factors, in your opinion, would need to be in place for this to happen?
If we knew of a child that needed a family, I am sure we would step in. We are also still pretty young. While terribly unlikely, I will not completely say that 10 or 15 years down the road, we might not want to have another.