Three important qualities of "mental illness":
1. An inability to effectively assess actual threats to one's self
2. Refusing to consider the possibility that you might be wrong about something
3. Having relatively little ability to understand one's self

One can by sympathetic to a lack in 1 and 3 but a lack of 2 leads to/ensures the continuation of 1 and 3. And it's very hard to be sympathetic to closemindedness.

Hard but not impossible. People want to be happy. Deciding that they must believe certain things in order for that to be possible is understandable. A lack of happiness can kill just as easily as "mental illness" can.

There is that, occasionally. But at other times it seems to be a sort of power struggle, where one person just wants to "win" no matter what. That if they admit they're wrong, then they are forever subordinate to the other person. Their mind, forever inferior.

The reality is that all of our minds are woefully falliable(sp). But to face this fact, is sort of awful for most people. To face how shoddy our own minds really are, can lead to a nihilism.

"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents." Lovecraft

Yes, merciful. But to realize that the above is really actually true, is pretty awful. Rather than face it, we're generally quite closeminded.