Is not having children selfish?

Catherine
I am 23, and I do not want children. I've talked a lot about it with friends, and although they support me, I have yet to find a female that really understands how serious I am. A lot of the time, people tell me that I'm too young and that once my "biological clock" starts ticking, I'll want kids, but I know myself, and I know I do not and never will want children. My last 2 relationships ended because I don't want kids. I simply do not relate to or understand them or have any maternal instincts. I've worked hard to reach where I am in my career, and that is way more fulfilling for me personally than children ever would be. So what are your thoughts?