I believe they are born that way. Something in their brain. My wife has a transgender cousin and she recalled he was playing with girls and acted like a girl at a very very young age.
Transgenders exists in pretty much all cultures around the world. Its an illness they cant do anything about it like you wouldnt make a down syndrome person normal you cant make them normal.
Actually, there are scientific studies that show that transgender people tend to have atypical brain structure. For example, some transgender people who were born male but identify as female show a slightly feminine brain and vice versa. But still that doesn't fully explain gender dysphoria or cannot be completely attributed to that.
Gender dysphoria can be caused by a combination of factors, among them enviromental factors. Many times gender dysphoria can simply go away.
Did Kroc feminise her face, by the way?