1. Because I am. I am intolerant of things like lactose, gluten, murder...stuff like that.
2. To make a point. There are things in this world we should embrace, things we should just accept and things we should merely tolerate. Then there are the things which should not be tolerated. I am intolerant of those things.
I'm not trying to say that tolerance is bad, but I am trying to say that it, as an ideal, is overrated, glorified and worshiped without regard to exactly what the object of our tolerance is. The way "tolerance" is used in the world today, it has become a synonym for embracing and accepting, not just recognizing and respecting.
I label myself intolerant because tolerance isn't all it's cracked up to be. We are all intolerant of something, even if it's just intolerance.