I think it could be for a number of reasons.

I think people in general are afraid when they see something "different" out of their realm of "norm". Instead of just accepting that people are different they feel the need to mock it for whatever reason. I personally beleive that the ones that choose to mock are lacking in self esteem. Somewhere along the lines it brings them pleasure to make someone feel as bad as they do. You know, the misery loves company saying.

As far as people passing judgments or making comments about people with mental illness, I just think that is purely out of ignorance. I still truely believe that their are people out there that think it is "all in our heads". They reall dont beleive that it is medical. No one wants to understand what others go through, they dont care.

It is the ugly truth to this not so nice world we live in.

I have always brought my children up to know that it is wrong to mock or tease others. That is not to say that they never do it because they do. When I see it happen, I question them as to why. It is usually because it is something they are not familier with. I explain the best way I know how and that is usually the end of it.