Because the medical system works like this, in order to get treatment you must have a diagnosis. If you don't have a diagnosis, you can't get treatment. That's just how the system works.
For example, the ICD is the international compendium of diseases, published by the World Health Organization. Discussions about the new version of that are under way now. That will come out in 2015. Certainly for most diseases, those are the codes that are used. When a doctor wants to bill OHIP in Ontario, he uses codes from the ICD for pretty much everything.
It's not written in stone yet, but the new ICD will probably categorize transgender or transsexual experience as being a physical disorder.