Question

why does a mic create that shrill sound sometimes that is unbearable???

Answer 1

It's called mic feedback. The mic picks up a sound from the speakers, sends it to the amp and the to speakers again. So, it becomes a continous loop, unless you move the mic away from the speaker.

That's why it gets louder and louder if you can bear to keep it in the "loop". Since the mic is sensitive, it will pick up frequencies silent to humar ears and then when you amp it, it is a very shrill noise.