What is special about mental health?
Probably that it does not show a clear pathology, like e.g. a broken leg. What's "broken" is inside a person's mind. If it's visible there is a question of current research. Researchers increasingly find biological causes for mental health but in the daily practice mental health clincians have to rely on what their patients tell them, pick up the clues from their behavior and base their diagnosis on that. Also the question if and when a patient is cured is harder to answer. There are no tests you can run, you cannot look if all the cancer cells haved died or if the leg is mended again. Again you will have to rely on personal judgement.
The third big difference is stigma. There is no stigma attached to having a cold or a broken leg, but there is a stigma attached to having a mental health problem. Immigration forms ask about a history of mental disorders in the same section they inquire about a possible Nazi past. There are estimates that one in four people will suffer from a mental health problem within a year. But how many do not seek treatment out of fear of stigmatization?