Why does my serpentine belt squeak all the time?

I have a chevy 5.7 with an auto tensionor. I replaced the idler and tension pulleys, but it still squeaks. I know it's the belt because when I spray something on it the noise goes away for a while. No matter what I spray on it the noise keeps coming back. So what makes it squeak? and Will getting a new belt fix this for good? The belt ribs are cracked, and the backside is shiny.