If we take a more gender critical outlook instead of a born this way outlook, I suppose it could be early negative experiences with gender roles, some traumatic body experience, maybe painful circumcision, maybe it was having gender policing parents who expected their "boy" to act like a boy and would enforce gender roles. Maybe it was thinking that femininity was aesthetically nice. Or wanting to increase my proximity with femininity because I thought it was 'pleasant' or a 'good thing for a person to be' in my mind. Maybe I looked up to, and wanted to be more like my mom than my dad. Maybe it was a curiosity about female anatomy. Again, I just don't know, because the want and feelings go back farther than I can remember. I struggle to remember much before I was seven years old. But what I do know is that it goes too far back for me to truly remember much of anything and speculate very well about any social causes. It just feels like I want be biologically and in some ways, socially, a woman, for reasons not entirely known to me.

This seems a good enough explanation to me, if we can take [female gender identity] as A Thing, rather than [female] [gender identity].

Our brains are formed by life experience and yes get stuff gets baked in early on from various experiences that even if we could recall with 100% perfect clarity later, doesn't mean we can just "undo it" by the realization.

There are people who, for whatever reason, intensely desire to be the sex they were not born as (and as that is not possible, the next best thing is to mimic it to an extent that they are perceived by random outsiders to be that sex, and "treated accordingly"). To me, that defines trans. Doesn't matter necessarily how they got that way, it is what it is. We can only move forward from where we are, we can't reverse time, so the only question is, how to relieve the symptom now.

People with a simple phobia, it's similar. They have fight-or-flight actual animal panic response to some object. They know it's silly. Plenty of them even know exactly how they acquired the damn thing. Doesn't matter at all - it's baked into the lizard brain, it's not just "I don't like spiders" type popular conceptions, it's actual "I recoil from this the way you recoil from a flamethrower to the face" reaction, without thinking, and if observed, outsiders will say "wow haha you jumped so fast that's freaky you were like superhuman" or whatever.

Knowing that, I am 100% accepting of the idea that environmentally acquired ideas can get baked in so far that the person has no idea where it came from, can't explain it, and can't imagine life any other way. Happens all the time.

At which point the question is "so what to do?" and for some people, if the answer is "avoid my personal Kryptonite, because it's totally doable in normal daily life" that's fine, similarly "I'll get bodymods to do my best" is similarly fine. People will do what people gotta do.