Psychosis and genius go hand in hand. It is that same ability to extend one's brain beyond the scope of human thought necessary to achieve the mental leaps that often characterize genius that can as easily be perceived by those protectors of good mental health as an excellent reason to apply electricity to the frontal lobes.
It is also a fact of despair in this culture that passion -- true consuming passion -- is seen as a mental illness. There is no genius without passion and thus, to the retarded subhumans who play at diagnosis, there is no genius without mental illness.
An even sadder fact, if it were not so prevalent in the species, is the tendency to fear the unknown and to not accept it. A genius might be pointed out, as an example, to have a really shitty personal life, or be divorced three times, or be (gasp!) a closet homosexual. Although being lost in a strange city is extremely common (and would be surprising in its absence, in fact), many people took considerable and unpleasantly vicious humor in the fact that a group of Mensa members were lost in a foreign city. Would their scorn have been as acidic if the group had been ordinary folks? Tourists?
When we are confronted by an intellectual, emotional, or physical characteristic that exceeds us, perhaps even approaching superhuman, our desire to find a counterbalance increases concomitantly. This is not, obviously, a reflection of the fact that geniuses are "just ordinary humans", but I suspect more a case of our own feelings of inadequacy manifesting in an extremely common manner.
Me, I'd take it as a compliment.