The River by Gary Paulsen
Theme One: Having someone around is much easier than being alone.
Theme 2: Being prepared for anything.
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.