Menstruation Is a Whole Cycle
When men only see their girlfriends and female friends for short periods, they don't realize how much their bodies are going through on a day-to-day basis, especially related to the menstrual cycle.
This kind of story is why boys and girls in school need to learn about how the reproductive systems of both genders work. Perhaps the reason men don't understand this is why they're so hard on women who complain about mood swings or PMS.
