What Perpich means by saying “You are the best thing you have” is that

you are the best thing. You are the only thing that’s going to help you,

your thinking is the best thing, when you think, about positive stuff,

not negative. The way I think this kind of thinking will help Brian is that

by thinking positive stuff he’ll get more motivated to do things that will

help him survive. Until his parents or the police find him. If he

thinks about negative things like he’s going to die or starve with no

food or water , that’s never going to help him. But if he thinks about

good thing and how to survive he might be able to stay alive for a couple

of weeks in the forest.