why do girls have menstrual cycles and boys dont?!?

ANDY'S ANSWER: Girls are the ones that carry the eggs and have the whole packaging system to incubate a baby until it's ready to be born. Every month a girl's body gets ready to make a nice 9-month home for a fertilized egg to grow. If the egg that comes down is not fertilized, the girl's body disposes of the egg and unneeded supplies and starts the process all over again to get ready for another egg next month. That, in a very simplified form, is what a menstrual cycle is. A period is your body flushing out the unfertilized egg and its unneeded nourishment.

Boys just the carry the sperm that fertilizes the eggs and they expell their stock every time they ejaculate.

It DOES seem like the work load for making a baby is a bit unevenly distributed, doesn't it? But it's a system that seems to worked well for a very long time.