The River by Gary Paulsen
Theme One: Having someone around is much easier than being alone.
One reason I think this was the theme it because it was easy for Brain to talk to someone instead of just talking to himself. The reason why is he would have someone to talk to and plan it more easily then being on his own and figuring out how to stay alive.
Theme 2: Being prepared for anything.
I think this is the theme because Brian was prepared from his last time being in the wilderness. I say this because he was ready to spend his life there if no one came to rescue him the last time he was there. The person that was with him didn't know very much and he wasn't prepared for a lot of things that could happen in the wilderness.
The same between these are when he went the second time with the other guy with him he was more prepared for everything because he was there for 2 months and survived. The person that went with him he was kinda prepared but he pretty much had Brian show him everything how to survive in the wilderness. So always be prepared for anything. It is always better to have someone with you if something goes wrong. It just makes it easier and plan out how to survive.