People characterize mental illness in one of two (not mutually exclusive) ways. Mental illness is:
1) a personal failing, a sign of weak character, and/or "Pull yourself out of it. Everyone has problems."
2) a sexy, fascinating affliction that makes the sufferer more creative and more interesting than the average boring person.
There's an excellent post that discusses the second point in terms of the public's treatment of Syd Barret here.