Why does a broken tooth hurt so much?

Hello, unfortunately, my right-hand back molar in my lower jaw is completely broken, and a piece has also broken off. I didn't notice it for whatever reason. Anyway, I wasn't having any pain for a while, and I thought, thank God, it had gone away, but today the pain suddenly came back.

Of course, I'll go to the dentist, but I'm just curious what exactly is causing this pain? Is it the nerve or the hole itself?

Thanks for your answers.