Max is a teenager who just wants to hide away in the down under "drooling in his comic books" (2.3) with "no fear of Gram sticking her head in the door" (2.1). Sure, we could chalk up the indifferent tone to the fact that our narrator is 13 years old. But you Shmoopers know better.
Max's attitude seems to be trying to force himself to not care about anything. Why? Well, when you care about something, you might lose it. That's what happened with his mom, and he's most certainly worried that it might happen again.
The words Max uses to describe himself also contribute to this surly and indifferent tone. He repeatedly refers to himself as a "butthead" (1.14) and says that his "brain is vacant" (2.3). This kid has serious self-esteem issues, and it comes through in his writing.