A girl I know always wanted to be a boy when she was a child. She even explicitly asked to have her gender changed through surgery in elementary school.
Nowadays she says, that her main reasons were that she preferred to do 'boys stuff' like soccer with boys rather than playing with dolls etc. with other girls. Plus she was really scared of the pain of giving birth as a women and thought she'd have to go through that when growing up. Up to this day, she still likes 'boys stuff', doesn't use make up, rarely wears dresses/skirts, didn't give birth.
So in her case specifically, it seems like it was 'just' the stupid female stereotype that she didn't want to live up to. She doesn't feel that gender disphoria anymore.
I guess independent of the gender we associate with, we as society should really object all these stupid stereotypes. Do what you like, dress as you like and be who you like, no matter the genitals you were born with. And let children know that they are free to choose their own path.