Someone once asked...
Someone answered with kindness...
The ease with which I dismiss another man's belief teaches me to suspect my own is as well. Perhaps they're operating under different ideas and circumstances than you. Have you tried to talk with someone you consider stupid, especially in a nonjudgemental, slow, and curious manner?
Another soul answered...
Because we can only notice stupidity on others, not on ourselves. And that's precisely what makes us feel intelligent. We are so goddamn stupid because "we are intelligent".