I think all of us have struggled with this question throughout our life.
Who we are,? why are we like this?, what caused this?, why me? .... are questions I constantly asked myself when I was in a state of unawareness and when I knew what it was, a state of denial.
Science may never find the reason, Religion will surely continue to say it is wrong, but do we need to know why?, there is after all nothing we can do to change history. Genetics, God, exposure to endocrine disruptions, who knows, but is it important to know?
Nature, throughout history again has a way of surprising us with new species, animals that are born different (and survive), that is a fact and will continue to happen... lets just accept it does.
We are born transsexual, that part as I said is history and we cannot change it, what all of us can do is shape the history to come, by accepting what we are, doing what we have to do to find happiness and helping others to face that challenge.
Buffy