The standard procession of life for women has always been to live for a while, then get married and have children. Throughout all of recorded human history, at least in the western world, this is the typical life. Of course, it's changed; from the late 60's on, women have started earning their degrees and getting jobs. Still, though, there's a certain societal pressure for women to get married and have kids.
There is, of course, nothing wrong with having or wanting kids. People who have children are how we survived this long as a species, and how we will continue to survive for millennia to come. It's an amazing thing to make a person, and an even more amazing thing to raise an upstanding, decent human being. It's an amazing thing to have the ability to make a person, which 10% of women ages 15 to 44 do not have. But having the ability to have children does not mean that I have an obligation, and it doesn't mean that I'm selfish for not wanting to exercise this ability. People aren't selfish for not having kids if they have the ability to.
A few years ago, my cousin and I would tell our grandma that neither of us wanted kids. She gave us the answer that most people give when you tell them that: you'll change your mind when you get older. Even though I've gotten this warning dozens of times, as I grow up I want kids even less.
A lot of why I don't want to have kids stems from the fact that kids are expensive. Because of our economy, having kids has become almost unrealistic. It's partly my fault; my major doesn't really give way to a profitable career. That leaves me, and many others, with a toss up: follow your passions, or have children. For a lot of people, including myself, passion takes precedent. But the world is expensive for most people. Student loans, the price of houses, the cost of living... It's too high for a lot of people, especially young adults. I probably couldn't afford to have children, and I definitely couldn't afford to give them the life that they deserve.
Along the same line, my career wouldn't be one that would be good for having children. The fact that I would constantly be traveling for research purposes, spending countless hours in my office researching and writing, and going on sabbaticals, leaves little room to spend time with my hypothetical children. Of course, my future husband would be able to take care of our hypothetical children, but that would be completely unfair to him.
There are more reasons than that, ones that reach into mentality. I was a camp counselor for two summers, and that basically deterred me from having children. Kids are incredibly stressful, and no kids are made in the same way. They all have their own personalities and needs, and it can be hard to keep up with them all. It's so, so hard to be a great camp counselor. Being a good caretaker of children is so nuanced, and it's so hard to get it right. Basically, I'm terrified of screwing up a child. I'm terrified of raising a person who is not a decent human being.
So no, my reasons for not wanting to have children aren't selfish. It's the exact opposite. I don't want to bring children into a life that wouldn't be the best life possible for them. I know that people with children will claim that I simply don't understand, and that everything changes when you have children. But I do understand, and I prefer to look at the realistic side.
I do, however, have some selfish reasons for not wanting to have kids. I would prefer to see the world, and experience things besides changing diapers and wrestling kids into a mini-van. I would much rather be able to spend a night reading or writing than waking up every three hours to a crying baby. I would rather go on dates with my hypothetical husband than worry about what kind of dates my teenagers were going on. My life is my life, and I don't have to give it meaning by having children.
In conclusion, my life is my life. If I want to have children, that's great. If I don't, that's also great. It's none of your business and, honestly, you're welcome for not overpopulating the world more.