Another aspect of mental illness is that it directly affects how you relate to someone as a person. It's a behavioral disorder, it affects the personality.

A person has heart disease or cancer. A person is schizophrenic, or bipolar, or manic, or psychopathic, or depressed.

It's like the aphorism "you have a body, you are a soul".

A person with cancer or heart disease may be concerned and occupied by the disease or condition, but it's not who they are.

I've had my own encounters with people suffering from various psychological conditions, and ultimately, it's simply very trying to be with them in many cases. It's worse if you're not aware of the condition at all. In several cases the information emerged later, with a very strong sense of "well, that explains a lot".

Worse is when the person turns on you, whether emotionally, physically, or by other means. And yes, I've had that happen as well.

I do have a great deal of sympathy for those suffering from such conditions, but my own self-preservation (and sanity-preservation) instincts also kick in after a time.

Much excellent discussion here, I might add, thanks all.