The origin of the desire

The desire to 'express oneself as' or 'be' the other sex seems to have many angles. Yes, we could look at the biological causation argument but that only explains one subset of people. What about those who are perfectly happy in their birth sex but simply need the outlet of expression?

In truth there is no way to know precisely which is why even individuals themselves can morph over time as they grasp their situation. They may have started off with a certain identification only to end up at a different place sometimes decades later.

One of the narratives I still see is the "I don't know why I am this way but I can't help it" which suggests a helpless swimmer carried along by a strong current. The desire to rid themselves of their gender variance may have been for the sake of leading a "normal" life which becomes more powerful than a primordial urge which isn't fully understood even by the person themselves.

The biological causation possibly loses ground for a person who begins to express variance after puberty but then we could talk about the predisposition being present but simply never acted upon until some watershed.

In the past we would talk in terms of mental illness or fetish (although some are indeed motivated by it) because the perfect binary was sold as being immovable and to diverge from it meant willfully disobeying the way one was created by God. Never mind that humans have far more elasticity to their behaviour. It simply wasn't allowed.