The basement is Max's security blanket. It's his safe place. When Max was four years old, he saw his mother brutally murdered by his own father. That would be scarring. On top of that, Max stands out among his classmates. He is much bigger than any of them. He calls himself a hulk. But it's not just his physical appearance that makes him stand out. Max is not that intelligent, so he is essentially the stereotypical "big, dumb kid."
All of those variables have helped to make Max quite withdrawn from just about everybody. Maybe he just wants to be left alone so that people don't tease him. Maybe he wants to distance himself from people as much as possible because he is terrified of losing a loved one again the way that he lost his mother. In either case, the basement "suits" Max because it allows him to be alone.
Symbolically, the basement represents Max's sadness. He only begins to come out when he meets Freak. And Max returns to the basement and its sorrow when Freak dies.