It is a mental disorder (Gender identity disorder (GID) or gender dysphoria)
Let me try it this way, it may be some sort of error in the human “base code” like homosexuality is. If we look at evolution and the purpose of life itself it is to survive and reproduce. If you are now attracted to your own gender you cant reproduce, there for an error or call it a mental disorder. (Note I don't have any problems with homosexuals.) A similar error could be the reason why some people think they are born in the wrong body (why is it an error same reason – no reproduction). Lets say male body with a female brain. Sometimes medication can help sometimes a surgery (Note about 20% regret a surgery and about 40% attempt suicide) Everything else from “gender fluid” to the “otherkin” a special snowflake symptom of narcissistic arrogant attention whores. Like “transtrenders” (Only transsexual for the attention). Does this help?