Question

Why does menstruation occur?

Answer 1

When in human female, egg is not fertilized, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilized egg. Thus, its lining becomes thick and spongy. This would be required for nourishing the embryo if had fertilized. 
However, this lining is not required any longer if fertilisation has not occured. So, the lining slowly breaks and comes out through the vagina as blood and mucous. 
This cycle takes roughly every month and is known as menstruation.