I think you have all made valid points. People fear what they do not understand. Sometimes its easier to distance yourself from somebody dealing with mental illness than to try to help or understand what they're going through.

I also think it depends on what the individual can handle themselves. They may also be depressed or have another mental illness that causes them to withdraw from others who have a mental illness. It may be too much for them to deal with or they may be isolating themselves from everything.