In chapter 5 of Hatchet, Brian's starting to reminisce about the past. Up until now, Brian's had to deal with the many practical challenges of surviving in this hostile wilderness, and so hasn't really had much time to think straight. But now, in the wake of recovering from his illness, his thoughts turn to Mr. Perpich, his English teacher. Brian greatly respected Mr. Perpich and always enjoyed his lessons, not least because of his infectious enthusiasm.
But more than that, Mr. Perpich inspired Brian with his mantra of positive thinking. He always used to encourage his students to stay positive and motivated, to realize that they themselves were the most important asset in their lives. Mr. Perpich's powerful message is especially relevant to Brian in his current predicament. He needs to stay positive, switched-on, and focussed if he's to survive this terrible ordeal.