why does spaz go back to the stacks

What chapter are you referring to? The Stacks is where Spaz first meets Ryter and Little Face. They are described as a kind of hobo village where no one really owns the land, but instead they set up temporary shelters and squat there because they have nowhere else to go. The stacks is made up of hundreds of rows of concrete boxes stacked on top of each other. The dregs of society live here, meaning those that have nowhere else to go. The area smells badly because there are no toilets so people have to urinate on the ground.