Should gender dysphoria/GID be considered an illness?Watch
I imagine that addressing it as an illness may make transgendered people feel offended, like they're being told that there is something inherently wrong with them because they don't fit into the neat little boxes society values so highly, and may cause greater psychological harm to someone who is just beginning to discover who they are and finding out that they're considered to be sick because they don't feel they are the same gender as their body would indicate. It can lead to a lot of self-hate, which is arguably even worse than having other people hate you for your gender identity.
I do wish that more people would consider the fact that there are no boxes. Or if there are, they fit EVERYBODY EVER into them. It's explained well in this video (which also covers sexuality) which I hope is informative:
PS: I took a survey on TSR, and when they asked for gender, they did the field as one that you could write in rather than having to select one box, so people could exactly describe which gender they were (or not, in the case of agendered people). It felt like a small victory.