I have not been around much these days, due to time constraints, and although this thread caught my eye a few days ago, I have not yet had time to read through the many comments. I'm looking forward to doing so, but I'm going to write my own response first.
I was not the type of young woman who dreamed of being a mother. I didn't play with dollies as a child. I don't remember thinking about it too deeply, but although I wasn't dead set against motherhood, it just seemed so, so far off in the future, and not something important in the here and now. I would decide later. Much later.
But I did have them eventually. I think the main reason was that I married a man whom I knew would make an excellent father. We had been together for many years -- nearly a decade -- before we decided that maybe having a baby would be fun. At first I thought I would take a brief break from my career and have just one, and if it went well, I would consider a second.
I was young for my peer group, only 31, when our daughter was born. I know this isn't particularly young for a first-time mother, but none of my pre-baby friends had kids yet, and most of them lived back in Vancouver anyway and I was starting a new chapter in California, where my husband had started a new job. I had no idea what I was doing. I felt almost laughably incompetent. But, as it turned out, it WAS fun. Enough fun that, four years (to the day) later, I gave birth to a son.
Like you, what I was NOT prepared for was the whole new world of worry that comes with being a parent. Stuff happens, bad stuff, that I cannot foresee or prevent. I cannot protect my children from everything, and even if I could, I shouldn't, because I want them to experience life fully. I try not to let it overcome me; life is for the living, and when your number's up, it's up. But I have come to realize that the worry is not going to stop when they're older, when they are adolescents, or even when they're adults. I'm going to have a constant maternal antenna up, paying attention, cranking out low-level anxiety, for the rest of my life. I don't know that anyone can really prepare you for that.
In retrospect, I think my real reasons for choosing to have a first child were as follows:
1. Husband who was clearly cut out for fatherhood
2. Biological urge, subtler but just as real as the urge to eat, sleep or have sex
And if I'm honest with myself, my reasons for having a second, after experiencing motherhood, were different:
1. Highly positive experience with first child
2. It almost seems silly to say this, but I realized that a child gives me a chance at immortality that I might not otherwise get. The only other way to live on after you are gone, and all those who knew you personally are gone, is to contribute something to the greater culture that puts you, either through greatness or infamy, in the history books. And although I can make a positive difference to the people around me while I am alive, I am unlikely to make a lasting cultural contribution. I am not religious or even a tiny bit spiritual. I cannot imagine how a person's thoughts can possibly survive the death of their brain. Once you're dead, you're dead. There are only two ways to live on. Children are the simpler option.
3. Realization that a child gives me a real, biological connection. Most people take this for granted, but I was an adopted child, and while I love my (adoptive) family and have no desire to seek out my biological one, it's somehow wonderful to know that I have an intimate relationship with two little people to whom I am also biologically related. My kids are the ONLY people in my life who fit that description. Without them, I am a genetic island. My children also deepen the connection with my husband's family; I am now permanently connected to them, too, in a way that I wasn't when I was a wife but not yet a mother.
Phew. That was long. Now I'm going to read everyone else's comments!