Maybe having periods is punishment for not getting pregnant and giving birth to a child - a gift of Mother Nature! They should go *pop*, grin and hold baby proudly in their arms! And then get pregnant again so they don't continue to be punished.

Of course, maybe the pain from childbirth is just payback punishment for getting out so many periods. It's like school - if you skip your periods, you'll get punished!

In all seriousness, if you treat girls nicely then most of them won't get that angry at you even when they're on their period.