The world isn’t completely honest about being a parent. There is plenty that they miss; plenty that they over-simplify and some that they get spot on.
Its impossible to describe the reasons for wanting a baby. There’s plenty of statements you can utter and each will be different for each person.
Perhaps it is a primitive instinct that kicks in. The need to continue the species. Perhaps there is no hiding from the maternal, no matter how much of a tomboy you were (or are). Perhaps wanting a baby is a natural extension of showing the love you have for another; the father of that child.
Maybe its because you didn’t feel love any where else, but crave the love to be found in a babies eyes. Maybe it is the only way to stoke the fire of affection of the man that you love. Maybe he will only stay because of a baby and you can’t bear to do this life without him.
Either way, when you get right down to it, you want a baby because you just do.