There’s a reason for everything. For you there are two, a physical reason, and a psychological one.
To explain the physical let’s start with the decibel.
The decibel is how we measure loudness. By which we mean sound pressure, the amount of force per square meter the sound is producing.
0 dB is the threshold of human hearing. 10dB is 10 times the threshold. 20dB is 100 times the threshold, and so forth.
The decibel is a really useful scale if you’ve got a microphone that responds the same to every sound, but our ears aren’t like that.
Our ear canals have resonant frequencies. You know when you tap a glass it rings, that note is its resonant frequency. If you played that note loud enough the glass would shatter. When wind around the Tacoma Narrows Bridge started blowing at its resonant frequency, it brought the damn thing down.
Our ear canals exhibit resonance in the 2000-4000 Hz range. Things in that range get amplified in our ear canals and sound much, much louder to us, which is unpleasant. Nails on a chalkboard has a lot of overtones in that range.
But why are our ears set up like that. Scientists have found that those noises trigger a stress response in us. Well it turns out an infant’s cry has a lot of overtones in that range too. So it could have helped keep us alive. We’d want to protect our own. Or shut them up, whatever. Interestingly, if you tell people that the noise is part of music, they say it’s less unpleasant, even though they’ll physically react the same way.
People are weird.
Lnr: I don’t know if it’s still true for me, because your hearing range tends to get narrower as you age, but I could hear tones way into the upper registry when we did an experiment in physics class. It made me feel special right up to the point I had to leave my friend’s kitchen because the noise of her mouse deterrent (which is at a Hz that humans are not supposed to be able to hear) gave me a headache. Fun times.