It's something hard to understand. Inside she's a female (this has been scientifically proven but I don't have any links), her brain functions like a female brain does, she feels how women feel, but outside she's a man. That means she's transgender. Who could be happy in the wrong body? So she has to go through the following legal process if she wants a sex change:
1. Start dressing like the opposite sex and living like the opposite sex 24/7 for 2 years while taking hormones.
Hormones will help her body stop growing hair, her voice will be more feminine, her skin will get softer and she will develop breasts (possibly not that big so that's why a lot of transgender females get implants).
After those 2 years going through all of that while going to psychological therapy at the same time then she's able to have a sex change with recomendations from her doctors saying she's mentally ready to do that.
Kim Petras had been taking hormones since she was 12, she was living like a female since she was 12 so...What's so shocking if she needed a vagina like the rest of women in the world do?
It is a personal choice because some trangenders don't want the sex change and some do.
I recomend you watch this video from a trans female called Calpernia Adams. It's a bit long but it worh it, it's really funny and it's called:"Bad questions to ask a transsexual":
http://www.youtube.com/watch?v=DjqsB1huDxg&feature=PlayList&p=EDA5242E79BA9150&index=0&playnext=1