To answer anonymously, you can write on my Admonymous and I will share the answer here
I've always been enamored of babies, but I've known and asserted for as long as anyone can remember that I don't want to have children. My sister has known and asserted for her whole life that she does want to have children. So to some extent it feels innate. When I was a kid I was told "don't say you don't want kids, that could easily change when you grow up." But I'm in my mid-20s now and in a stable relationship, and I've never wavered on that point once in my life that I can remember. I still have ~15 years left to change my mind, but I'd be fairly surprised if I did.
Both my sister and I did a fair amount of babysitting, and while she found it very rewarding (and went on to work with kids professionally), I found it extremely draining and a constant struggle. I have fatigue problems and had lots of trouble keeping up with the kids; I constantly second-guessed how stern or lenient I should be with them; I was garbage at telling stories of any kind; I couldn't keep running when they wanted to play tag; my arms got tired bouncing a crying baby; when one girl smeared her poop on the walls I just wanted to throw up my hands and cry. I often say that I love babies, but only because I can give them back.
I think there are a lot of things on top of that that make me a bad fit for parenting. I need a lot of sleep and a lot of alone time. I don't always have my emotions very well under control and I know from experience how scary that can be for a child. I have a lot of things that I want to accomplish that don't involve children, and I'd certainly have to make major changes to the way I live to accommodate a child. I also date people who don't want children, so suddenly changing my mind on that point would be a dick move.
Another big one is pregnancy. While a lot of women my age (including my sister) go a bit crazy with the desire to physically be pregnant, I find the idea absolutely horrifying. I have enough health problems and hypochondria without adding pregnancy (and its after-effects) to the mix, and there's also something I just innately find quite disturbing about it, to the point where I subconsciously avoid friends when they're pregnant. I'm in a play right now where my character gets pregnant and the idea of donning a fake belly was so creepy to me that I could barely even talk about it, and even gesturing to my normal stomach and pretending there's a baby there onstage makes me feel really uncomfortable.
So those are some of my many reasons. I think you can see pretty clearly that nearly every single factor points in the same direction, although there are two exceptions - (1) I really love the feeling of a soft warm baby in my arms, and (2) I am descended in an unbroken line from the very first self-replicating molecules, and it feels bad to, like, spit in the face of my fishy ancestors by stopping the line here. But those two things are not nearly enough to outweigh the others. So it's a no from me.
My genes followed their programming, allowing my mind to make up rationalizations on the way. Here are some clever rationalizations for you:
Having kids is like cooperating in a multigenerational Prisonner's Dilemma. If you think your existence is a good thing, remember that someone paid the cost of that. Paying the cost of having other people like you exist is how you reciprocate.
If you are awesome and your partner is awesome, by making kids you make the humanity and the entire world more awesome!
Being a parent is a powerful experience that will satisfy your curiosity in many ways. You will have to assume "heroic responsibility" for your kids. You will get the opportunity to observe how utterly absurd is the "blank slate" theory. You will experience altered states of mind as a result of sleep deprivation. Women get the opportunity to observe the massive impact of hormonal changes on their minds. Parenting is one of the most powerful human trips.
Watching little humans grow up and gain skills is emotionally satisfying. Even more if you can give yourself partial credit for the success.
The book "Selfish Reasons to Have More Kids" was a bit of an influence. The quick summary: People often overestimate the downsides of having children, people often underestimate the upsides of having children, people overestimate the marginal benefit of more labor-intensive methods of parenting, therefore maybe you are underestimating how many children you should have (including underestimating the benefit of tradeoffs where you have more children but use a less-intensive parenting style).
I think choosing to raise a child rather than not will probably make me happier when I'm older, even though it's not very pleasant a lot of the time currently, and there is the constant additional exposure to the risk of terrible tragedy. It gives me a reliable source of significant responsibility, which overall I value. I like that I'm playing a small part in creating the next generation of humans (and thus in creating the whole set of future humans), I think that's cool, though having children is not the only way to do that.
I think that human beings are very psychologically flexible, and I haven't been persuaded by arguments that it's not the case that the vast majority of human beings have lives worth living. I also am not persuaded by arguments that favor autonomy to the extreme that it's bad to bring someone into existence because they had no choice in the matter. While I don't think this amounts to a moral imperative, I think having children is a good thing, if the quality of parenting is even minimally acceptable. Overall, I think having and raising children is good for parents but primarily it's good for the children (and, indirectly, their descendants).
That's easy: they were there.
You didn't ask about having children, you asked about raising them. I have dumped so much time and money into my nieces and nephews that I should be listed somewhere on the birth certificates.
I will never have biological children of my own for many good reasons, foremost being I have significant mental illness. I'm not about to pass that biological curse onto anyone else.
The main reason is because we (wife and I) find that our well-being is highest when we're around family and having kids is a great way to increase the chance of having family around for the rest of our lives. Yes, there was the well-known short-term hit to our happiness during the kids' youngest years, but we were going for long-term stability for our well-being and life satisfaction. Of course, this wasn't the only reason. We also knew we had the type of marriage that was likely to remain stable/satisfying and we knew we could give our kids' a great start in life. So, it was a good bet that the kids themselves were likely to live healthy, satisfying lives. For us, it was a pretty easy decision, especially once we were able to find our cohort buried in the research on parenthood and life satisfaction.
I've always loved being with kids and wanted my own to raise someday ever since childhood. It just feels like an inherently good and meaningful thing to do for me.
The contraception didn't work and it was too late for abortion (should we choose it as an option)
In the OP, I invited people to submit answers anonymously to be published here.
Pure selfishness, in my case. I see kids as a burden, not a blessing. No doubt these are values picked up from my parents, but they are strong enough to override my biological and social programming. Your mileage may vary.