It's largely biological. Add in the constrained choices permitted by a culture that denies biology for ideological reasons, and even if it were a matter of choice the alternatives to transition are less than appealing.
The 'choices' I am presented with reduce to three possible longer term outcomes.
1) Socially Acceptible: Suicide. I put myself out of societies misery. A few folks say "Tsk, tsk..." A few mumble about how it's so sad, but he was so selfish. Many say "good riddance" and go back to their cheese nachos and watching pro wrestling.
2) Socially Tolerable: Self Medication. Alcohol, opioids, Doc Feelgoods prescription mill. I become invisible to society, a drugged zombie at home, or in an alley conveniently out of the way. A few folks say "Tsk, tsk..." Some may want to bust me and send me to prison where I can learn useful skills such as burglary or meth cooking.
3) Socially Unacceptible: Transition. I live. I am happy. A few folks say "Tsk, tsk..." Some will say I'm being selfish. Some will say I'm going to hell for violating a bad translation of a tribal law of some ancient nomads. Some may offer to send me there. Using public restrooms may be dangerous or illegal. But, I live. I am happy.
Nice set of choices I have there. Is it any wonder that given the evidence for a strong biological component to my nature that I accept that over ideologically driven nonsense like "all gender is choice"?