> If you choose to have your hand cut off because you think it would be awesome,

Does that cut mustard? Why not?

That's begging the question. The point is, "Who decides what a good reason for this is?"

After all, it seems self-evident that nobody would seriously ask for these things if they themselves didn't believe they had a good reason, yes?

The argument proffered through is that it is necessary for their mental health. Which then begs the question, what about people who compulsively get plastic surgery? Which is exactly why the DSM carefully says that "gender nonconformity is not in itself a mental disorder. The critical element of gender dysphoria is the presence of clinically significant distress associated with the condition". Body dysmorphic disorder on the other hand is itself pathological, so continued surgeries do not rank as humane treatment.