Who decides what is and isn't a mental illness? Why do they do it? What makes a mental illness, anyway?
Human beings, professionals, propose mental illnesses. They do this because they want to help people. A mental illness will have a label, and it will have some related behaviors and thought patterns.
And what's the point of that?
These mental illnesses are things that impede a person's ability to function. We have to find ways to improve life for the people afflicted. If many people have similar conditions, they may benefit from similar treatment, and that's why people get labelled with mental illnesses.
It sometimes seems like the purpose of things like this is lost on some people, but 'why' a field of study exists should be the first thing anyone learns about it. If you don't know why something is talked about, you should try to find out before talking broadly about it.
We want to help people, and with that in mind, it is not fair to call people mentally ill simply because you don't personally like or agree with their lifestyles. It is especially unfair to people who are suffering from actual mental illnesses. Nobody should be lead to believe that illnesses are diagnosed out of spite or intolerance for the afflicted.
'Being transgender' is one thing that is not a mental illness but that many people will happily call one out of ignorance. Sometimes it's malicious, and sometimes it's just the result of confusion. Gender dysphoria is considered a mental illness, and people with gender dysphoria tend to be transgender, but there is still a difference between being transgender and gender dysphoria.
Gender dysphoria is about the psychological stress of having the wrong body. It is diagnosed so that the dysphoria can be treated, and it is treated with transition. Most people will get great relief as they transition socially, with hormones, and/ or with SRS. In other words, treatments for gender dysphoria work but the people who go through it are still transgender.
There are therefore many transgender people who have been through treatment and now feel no or very rarely feel dysphoria (Hormonal therapy and sex reassignment: a systematic review and meta-analysis of quality of life and psychosocial outcomes). They may now fit into society better than they did pre-transition and they may now be happy (Site with many links to studies showing HRT, being out, and SRS improving life). There may be nothing to treat as a result of all of this, and a transgender person in that position is not mentally ill.
And to a select few people: It's not a conspiracy against you. It's the efficient allocation of resources to the people who actually need help. Psychiatrists don't exist in order to police people's self- expression into alignment with delicate fringe- conservative principles.