I think many see it in the category not of social expectations, but of biological ones. There are all sorts of odd, extreme, and more or less rare medical conditions that cause people to have some parts of their organisms develop in ways different from what would be considered "a healthy human", that range from inconvenient through making it incapable to live independently up to "will die without highly advanced constant medical supervision, and even with it, probably won't live anywhere near an ordinary lifespan".
Such biological basket cases are usually seen as illnesses to be cured to as great degree as possible.
But what if such rare and massive biological issues develop in such a way that they affect the brain and the rest of nervous system, rather than merely such obvious cases like missing/ malformed limbs or internal organs?
I understand the reservation about medically "rewiring people's brains" in general, but for the sake of fairness, the same reservation would have to apply to any psychiatric/psychological and even some neurological interventions. After all, those include, or in former cases, are purely focused on changing the patient's behavior, probably in the direction of what is considered healthy, ordinary and normal.
In some of these cases the patient may be an adult and capable of making such decisions for themselves. In the other extremes, the patient may be struggling to make decisions on what is the proper time and place to relieve themselves, nevermind complex medical-ethical decisions. And in the most complicated, the patient may be able to make decisions, but have decisionmaking ability impaired by the very medical issue in question, much like someone who is drunk, but indefinitely, at least until an effective medical intervention, in cases where such is possible at all with current state of medical science at all.
So, what to do with such cases in general? Transgenderism is hardly the first or the only one.
Oddly similar discussions are happening regarding autism, Down syndrome, ADHD, and the so called "deaf community", each case with its individual quirks regarding the medical details, effect on the afflicted, and effect on the society in general.
The one thing these cases share in becoming more of a public discussion than other mental illnesses, is the question of the afflicted interact with society in general, rather than just their own personal struggles, and in turn how should the society interact with them.
The "deaf community" case is closest here, as if people choose to not (partially) cure their affliction and live with what is traditionally considered a disability, should it still be considered a disability? Should they be pitied and helped by charity/social security, and benefit from special laws to protect them and their interests, if they think that their difference from the norm is not a medical problem, but an alternate and completely as valid and equal in every way to "the norm" identity,way of life, or lifestyle, despite the obvious and non negligible ways in which it disrupts related everyday interactions with the society at large?