ooohhh oohhh... Can I try to answer that one too?
Some people believe that if they change 'enough' of their body's sex markers to resemble the other sex then they can claim to 'be' the other sex. This means that while you cannot change your chromosomes, you CAN fill your body with artificially produced hormones at the level found in the opposite sex and declare that you are 'hormonally the other sex'. ie. your hormonal sex marker has changed.
You can surgically ( or hormonally in the case of breast tissue) change your physical sex markers. ie. remove breasts, ovaries and womb and surgically attach a surgically constructed body part to resemble a penis or invert a penis into an artificially constructed cavity that a person might call their 'vagina' but is nothing at all like a vagina. But it may be a fuckable orifice like some male people consider a vagina's purpose in life.
You can change your facial structure too. Male people can have bone shaved and cheeks implanted, and face pulled up and even the gap between the nose and lips altered to more closely resemble a female face.
So if you have all these procedures, they believe they have done 'enough' to some how deserve being accepted as being female.
Also, some people believe in post modernist philosophical theories that work along the line that 'if you say you are x, then you are x. Because who is to deny your reality'. So therefore magically saying you are female, makes you a female. Because after all, you should be able to be a female with a penis. Because that is your personal definition and people must accept that.
How did I do?