I would assume because you don't feel the best and you may be worried about people noticing. I kow when I get my period I just want to curl up in my Pjs and do nothing all day. Not only is it uncomfortable, but I always feel dirty and unattractive. I think what you are feeling is completely natural.
Periods are a pain, cramps, your mood is all messed up, and your always having to go to the bathroom, and on top of that you have to worry about not stainning. I don't blame you for staying home.
Because we generally don't feel good and feel tired during our periods? I used to hate going to school during that time of the month, because I felt gross and fat and moody.
maybe you feel like you might bleed thru. thats always embarassing, especially in school with all those rude kids.