Maniac does not accept Mars Bar’s invitation because he is not ready to have a family yet.
Over the course of the book, Maniac develops a friendship with Mars Bar. He is a strange kid, but then again so is Maniac. Maniac is not used to being around people, and at the time that Mars Bar asks him to come home in chapter 45, he is reeling from the reminder of his parents’ death as a result of the incident with Russell and the trestle.
All we sayin’—all she sayin’—is, you wanna come for a little, you know, visit? You want to? Well, come on, you can. That’s all. Don’t go makin’ no big thing, man. Ain’t no big thing.” (ch 45, p. 180)
Maniac shudders not because he does not like Mars Bar, but because he can’t imagine going home with him. Although Maniac does not want to go home with Mars Bar, he is unable to explain to him why he can’t go. This is because Maniac is unable to share his feelings. He is used to living alone, and on his own. He is not ready to become part of a family even though he is friends with Mars Bar.