Why do most people hate mentally ill people?

I have bipolar disorder and I've always been afraid to talk about it with people. They always jump to the worse possible conclusions about it. They assume that I am a mental case up and down. Here recently I've become way more comfortable with it and the more people that can be educated the less of a stigma. I know I might only be changing a few minds. But that's a few minds changed and when the next person they come across that is diagnosed with a mental illness they might not be so judgemental and more understanding.