Fun facts:
80% of all kids that "feel" trans, spontaneously stop sometime after the onset of puberty if there's no medical intervention (i.e. hormone blockers, etc) (source: Vanderbilt University and London’s Portman Clinic)
With psychological therapy, those who continue usually hit a wall, and once they break through that wall they get to the root of those feeling and it's typically due to an event in their lives (i.e. a parent or grand parent that liked to dress them as the opposite sex because that's what they really wanted has been reported often)
Also, it's been reported in various places that about 98% of all homosexuals were at some point in their lives molested, groomed, or outright raped by the same sex, usually someone older.
At the ages that this kind of shit happens, the brain is particularly malleable. This is the point that you learn the most and one of those things you learn is how to cope with traumatic events and assimilation is one such coping mechanism.
There's a lot of shit going on and a fuckton of research going into this. One thing's for certain: No one is "born that way", that much has never been proven.