Similar to marriage, having kids is the proof that one is finally completing his/her mission. This person is having a family. It isn't only increasing family population, it is giving contunuation to yourself and of another special person to you.
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Giving birth to a child is extremely important to a woman, because it is why God made her, to reproduce and give birth to her successors. It is actually, more than that, every girl wishes to have a daughter to make her be the perfect little girl or a son and make him the most handsome boy. The only way to complete this other objective is having kids or adopting, which is also a great way of accomplishing life's purpose.