I think that it really depends on the person. I personally have never really had this desire to have children. I think that part of me is selfish. I do not want to lose my body, my freedom, and so forth. Perhaps some of it is just because I am so young and I will "grow" out of it (or at least that is what people tell me).
I think you just know if you want kids. I have always been told that it is just this feeling you get like being in love. You cannot force it I do not think.
Again I do not have kids so what do I know

Jessica