Why does the sound of nails on a chalkboard bother us so much?

Chris - The only, and the best, explanation I've heard of this is that the frequency distribution that's emitted by the nails going down the chalkboard is very reminiscent, at very high frequencies, of distress sounds produced by other animals. This plugs in to the primeval response in all of us of an animal in distress. For this reason, we experience this sound with an "Ouch! I don't like it! I have to wake up and take notice!" reaction.