People don't understand mental illness. Some knowledge you can only obtain through experience. I believe the burden of mental illness is the case with this. If you haven't lived it then you're ignorant of it. It's like trying to explain the color red to a colorblind person. It's impossible to describe the color red if you can't experience it. Sure, you can measure the light wave lengths and other objective features but that's epistemologically different than the actual sight of the color red. The same is for depression or any other illness. It's easier to tell something is wrong if you break your leg. They can see that and know you can't mechanically get back up so its obvious you need help, but mental illness is a sinister thing that can't be seen with the visual eye. You can't point to depression, only behavior. It's a quality of our mental life that can't exactly be quantified so readily.