I have a handful of friends who are one and done-- mostly for financial reasons. For me, I want a boy and a girl. I want my children to have siblings because I appreciate mine so much and I want my kids to have family supports when I'm no longer around. I also want an insurance kid-- you know, in case I screw up the first one or something horrible happens to one. (Yeah, I'm that kind of macabre.)
But, that being said-- I'm a HUGE proponent of not having kids you don't want. So if you don't want more than one, I would never tell someone to change their minds or pressure them about not having more.
This! I thought I was the only one whose reasoning for 2 kids involved having a "spare". I know it's a horrible thought but you never know what could happen. When I had my daughter, I wasn't sure if I'd want another one (especially in the middle of those first few months). And yeah - it's surprising the number of people who ask this question when the first one is not yet done baking, or just out of the oven! Anyways after a while things got easier and...it started to sound good. I want her to have someone to complain about me to when she gets older. I want her to have someone that understands in the unfortunate event that I, like my mother, get Alzheimer's when I'm only 60.
That said I could totally see having only one, and I know that I would have been 100% happy with just having my daughter if I could not have had any more kids. It's totally a personal decision and I am glad that you have been able to make a decision that makes sense for you and that you have peace with.