As Brian progresses through his 50 plus days in the wilderness he keeps a mental journal of the things he needs to remember to keep himself alive. Brian continues to make mental lists of "things to do." He realizes at one point that feeling sorry for himself won't help, so he begins to try and do the things he remembers from books, tv, movies and teachers. As he learns a new skill or a new way to do something he makes a mental note so he can repeat the activity. Most of all Brian continues to keep track of his "things to do." In doing this, Brian survives, is found and returns home a much wiser young man.
In the novel Hatchet, Brian is stranded in the wilderness all by himself. He has no means of communication, no food unless he kills it himself, and no way of speaking with another human being. Because of this, Brian is forced to survive, learn the instincts of being in the wild, and test his ability to persevere. Since he needs to remember not only the survival techniques that he picks up in the wilderness, he also needs to remember and reminisce on all the things that he had before the wild, in order to stay sane.
For instance, when Brian learns how to go fishing by using a hand crafted spear like object, he notes it down in his mental journal. As read in the book, this act of fishing becomes successful and leads to many meals for Brian. This would not be possible had he not remembered it or acknowledged it in his mental journal.
Imagine if you were stranded out in the wilderness. You would do everything possible not only to remember the techniques that kept you alive, but also the ones that kept you sane.