I know, I know, it can be caused by various factors, like high ISO, poor lighting conditions, long exposure time, heat. (not that I know why the last two cause noise)
But where does noise REALLY come from? What is noise? What does it have to do with photons and photosites?
I think of it like trying to listen to a really quiet video or audio file on headphones or speakers. You can keep turning it up until you can hear what sounds are on that file. But you will also be turning up the background white noise