I think L&B's answer is spot on.

For a couple of years after DD2 was born I wanted another baby but for reasons that were purely selfish - I wanted to be able to breastfeed (my girls couldn't for health reasons). I wanted to not have PND again, or if I did to manage it a whole lot better and be the happy, self assured mother I wanted to be but hadn't been in the past. I wanted a healthy pregnancy I could enjoy. Because I couldn't see a reason past those, the answer for me was no. I felt that unless the overriding desire for a baby was purely for the baby, and not to "fix" things for others, then it would be wrong to have one. Another year or so down the track and my reasons changed - I saw my children running around and couldn't imagine not having another incredible little person in our family. Its why I get so peeved when I get the comments like "oh, you must want a boy" or "you must be so disappointed to be having another girl". I want a baby, an individual. Another little person to share my love and life with.