Why do we say congrats to first Period?

Question

Why do we day congratulations to young girls who get their first Period? When I started my period for the first time 15 years ago, I was so embarrassed and horrified! I didn't even want anyone to know. Any one relate to this?

Answer

Probably for the same reason why we congratulate women when they just had a baby. 
"Congratulations! You just willingly went through the most excruciating pain you'll ever go through and completely ruined your body in the process - you'll never be a size 2 again. Oh and did you SEE how big your vagina stretched? Man you almost got placed into the high percentage of women who's vagina tears all the way down to their ass hole. Maybe next time you'll hit that milestone. But good for you anyways!"
Likewise, "Congratulations! You have to deal with horrible, often debilitating cramps, hormone fluctuations (including acne), and OCEANS of blood pouring out of your vagina for the next 40+ years! You'll also have a ton of fun spending your hard earned money on pads, tampons, new underwear, pants and sheets. Oh! And with this milestone comes hair in awkward places, random crying, weight gain, stretch marks, and odd smells coming from your vagina. IM SO PROUD." 
This is how I see womanhood.

Answer

It's a milestone.

Answer

We congratulate someone because it's a big step towards becoming a woman and being able to reproduce.

Answer

because you are going from girlhood to womanhood.

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I've never heard of that. No one, but my mom even knew bc she had to start buying pads for me. I'm guessing if someone does congratulate someone for that it's bc they've reached a milestone.

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I've never heard of anyone being congratulated

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Never heard of that.

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My family was so happy that my mum made a special dinner and cake :) I was so embarassed :) but now I get it... it's like a milestone, you became capable of bearing children of you own, continue the family, so they're happy their genes can go on lol

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I feel bad about me bringing a girl into this world sometimes because I would have given her the curse of periods.. I don't feel it's celebratory time but a time of sadness because you just can't be cool and carefree anymore