Add in another voice saying "more please!" :D
Now WHO WANTS TO HEAR A BIOLOGY STUDENT RAMBLE ABOUT PERIODS because goodness knows I looked it up once when curled up on the couch with a heat pack against my lower abdomen. :P "Why does this happennnnnnn"
And the answer given in the episode was pretty good! But incomplete. Squiddy's research left out why the endometrial lining (the stuff that gets shed when a period happens; the uterine lining of mucus and blood and tissue and all that good stuff) in humans is so thick it's energetically not worth it to try reabsorbing it.
And the answer is; because human zygotes are freaking aggressive.
Once an egg and sperm join up to become a zygote and start dividing, that ball of cells has one particular job; secure nourishment for further development. So it implants itself in the uterine lining and burrows in, seeking a sweet delicious blood supply from the mother that will feed it for the rest of its development. In most mammals, it doesn't burrow very far, so they don't need a super-thick endometrial lining; there's plenty of time for the umbilical cord and placenta to form around the fetus so it's protected and nourishment is both secured and controlled.
Human zygotes, though, don't particularly know when to stop. They want AS MUCH OF THAT SWEET DELICIOUS BLOOD SUPPLY AS POSSIBLE, and would burrow through the uterine wall entirely if they could reach it. This is a Bad Thing. So, over time, selection favored the secretion of thicker uterine linings, which allow a little more of a buffer zone and time so that the placenta and umbilical cord can form.
Of course, building up a defensive layer that can still support life is quite a process. Maintaining it also takes a fair amount of energy. So basically, through biochemical mechanisms, if there is no zygote attachment detected, the body goes "whoo! Supplying blood to all this extra tissue is tiring. It's not supporting anything anyway. Hey uterine lining blood vessels, constrict and reel it in, would ya?"
But then without a regular blood supply, the parts of the lining farthest away from the uterine wall start to die.
You do not want dead and dying tissue to stick around in your uterus. That way lies infection and all sorts of unpleasantness.
So what does the body do? It sheds all that extra tissue. And lo, a period! :B
After the tissue is shed, the uterus takes a little breather. "Ahhhh," it says. "What a relief to not have to supply all that extra tissue! Why did we make it again?"
And then the ovaries say "HEY HELLO I'M ABOUT TO SEND ANOTHER FERTILE EGG YOUR WAY GET READY FOR BABY :D"
And the uterus goes "SHIT"
"POWER UP THE SHIELDS GO GO GO WE GOTTA GET THIS BUILT BEFORE A BABY COMES ALONG SO IT DOESN'T DIE AND SO WE DON'T DIE"
And the cycle continues. Until menopause. :B
And now you know! :D