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 situation.
Anyway, he left you alone in the house for those three days. We didn't live where we do now. It was a very large house. She abandoned me eventually, and they took me back to the old man. He'd sobered up enough to go home. I suppose you developed your fear of being alone then.
Another good quote comes when Rusty passes out, and Steve runs off for help. Rusty isn't unconscious for too long, so he wakes up and realizes that he is alone. He essentially panics and runs to find someone and anyone in order to not be alone.
I wasn't out too long, just long enough to scare Steve into looking for some help, so when I came to I was laying there on the roof by myself. I fixed that as soon as possible, almost running to the roof door.