Why would someone who is stick-thin firmly believe that she is fat, to such an extent that she dies from malnutrition?
Good point CoteDAzur.
Because that question is often asked. Why would someone go to all that trouble?
When you consider the difficulties with transitioning, particularly the surgery, all the trauma involved in coming out to friends, family and work, etc, the motivation must be strong indeed.
It's used as an argument for the 'born in the wrong body' narrative.
Anyone, with an ounce of imagination, would suddenly think, crikey, that would be like living some kind of nightmare.
But when you read more, the premise that the person is actually mentally female, just doesn't ring true.
What does come across, for many (though not all) is a strong self obsession that becomes evident very quickly. Plus an irrationality that is difficult to address.
I think it has a much greater correlation to conditions like anorexia, than anything else. You might as well ask why do the transabled have healthy limbs cut off? It sounds to me like very much the same thing.
And where does the anger come from? There is a lot of (narcissistic?) rage at the merest hint of dissent.