The problem with this thought is we cannot judge a person of whether he has a mental illness or not. Yet people do that, and that's exactly the problem regarding stigma for example.

Mental illnesses are titles written in personal medical files.
One way to cope with this is not to take mental illnesses as a whole, but to take it as a PART of the whole. Expecting the person to be depressed all the time just because they have depression is stigmatizing.
Expecting the person to have some symptoms of his acclaimed mental illness is starting to become more realistic, both for the benefit of the doubt and the fact that a person is NOT his mental illness, as I said above. The person is a living human being just like all of us, with a wish to live the best life possible