We are happy to have one child and only one. Both my husband and I grew up with siblings; he had a better experience than I did, but I can't blame my bad experience entirely on my siblings, nor did that factor into our choice to have one.
Some reasons we have just one:
We were 36 and 44 when we had him. Fast-forward to visions of his graduation: I'll be 54 and Dad will be 62.
We have a small house, small car, small life. We didn't want to have to sell or remodel the house or to buy a new car. Seems insignificant, but we do try to live with as little debt burden as possible.
We want to give our son a good start in life. Having one meant that we could afford a very good and loving preschool. We can afford to go forward with pursuing therapy for him and are still trying to suss out what's going on with his vision issues. This costs a lot of money and we aren't close to done walking this path. We'll hopefully be able to help him out when he chooses to pursue higher education or training in a career that can get him started. (Even if it's not his dream career, we would like to help him on the road to independence by making sure he has options.)
I had three miscarriages previously--two with the exhusband (blessings in disguise) and one with my husband now. Suffice it to say, the emotional risk in trying for a sibling was something I wasn't willing to go through again.
And this isn't a popular opinion, but we believe that our having a larger family shouldn't be on the backs of others. Many of the stores which offer great savings and low prices aren't always paying a reasonable wage to workers overseas. I don't want the products we buy for our family to come from factories which exploit their workers or use child labor. No other mother's child should have to work or suffer so that my kid can have 'cheap' anything. We know we can afford to do well with one child; I can't say for sure that we would be able to do this effectively with more kids.
Lastly, I was a nanny for years in between having preschools, and saw what a huge change going from one child to two was. My child is not an accessory--he's my life-- but I knew a long time ago that having two children wasn't a good temperament match for me, frankly. I'm far too introverted and my husband is far too busy with his work for us to pull it off without feeling stressed out.
Recently, my son has been asking for a sibling. I have a short job for a month, taking care of a little six-month old girl once a week; this job started yesterday and my son was a *wreck* by the end of the day, having to share me with her. At five, he thinks a sibling will be someone for him to play with-- I think this experience is quietly teaching him that his fantasies of having a brother or sister do not meet up with the reality. Having (only) one or having more is a big decision, and a very personal one for each couple. There's no 'wrong' answer.