Readers get two quotes very early on in chapter 1 that start to clue us in to the notion that Rusty is not comfortable with being alone. Take the following quote for example:
Like I said. My memory's screwed up some. If somebody says something to remind, I can remember things. But if I'm left alone I don't seem to be able to.
Being forgetful every now and again happens. Everybody experiences it, and it's normal. What Rusty is talking about is more severe. There is a hint at some kind of memory disorder or long-term symptoms of multiple concussions. Rusty admits to knowing about the memory issues, and it clearly bothers him. What's more important is that he knows that being alone triggers the memory lapses.
A few lines later, Rusty tells Steve that he'd been put in solitary confinement before, and the connotation is that it was a horrible experience. Later in the book, Motorcycle Boy explains to Rusty and readers exactly why Rusty might be so terrified of being alone in any particular...
(The entire section contains 2 answers and 598 words.)