Great question. There are so many possibilities. Some people are born mothers and having a baby will help them feel complete, but sometimes craving a baby is symptom of something, like insecurity in a relationship. If craving a baby is so intense that it's interfering in your life, or that you're very disturbed by it, you might want to seek a psychological evaluation.
You can answer that! How? By asking yourself questions. Do you feel you are almost too old for children & haven't been able to conceive? Are you single & have fears of or don't think one should have a child without both parents!? Do you envy a friend or family member who has a newborn? Are you being pressured to get pregnant? Are you just plain lonely & think a child would clear that loneliness. There are more, ask.