When I finished reading Stickeen, I started thinking about the pet culture. Granted, Muir does not keep Stickeen as a pet the way that many modern people keep pets, but the elements are the same. Animal domestication is an interesting aspect of our culture. We take little animals and bring them into our home, feeding them and sometimes treating them like our children.
All dogs, little Stickeen included, originated from wolves. Over the course of time, humans bred the desirable, somewhat more docile traits from wolves to create different breeds of dogs that exist today. Some dogs were bred with specific hunting or guarding traits, and some were specialized for herding.This is a great example of humans effectively taking something from nature and changing it to suit their growing needs.
I see dogs, Stickeen as a particular example, as being links between raw nature and the civilized human world. Many of the stories we have read present expeditions into nature as being something that allows an escape from reality. Many adventures seek out time in nature to rejuvenate, or to get in touch with their natural roots. It is possible that one reason why so many people like to have pets around is because they can be a constant bit of nature and the natural world that can fit into daily life.
In Muir's Stickeen, he even specifically says that "through him as through a window I have ever since been looking with deeper sympathy into all my fellow mortals." When Stickeen accompany's Muir and shares in his adventure, the ultimate outcome is that Muir has a deeper insight into the world, both in nature and in the strict human world. Stickeen seemed to somewhat link Muir even more effectively to his natural surroundings. As human as Stickeen could be, he also was very at home in the wilderness. He bridged the gap between humans and nature and helped Muir to do the same.
Fundamentally, I think this is one of the reasons why we bring cats and dogs and bunnies and lizards into our homes. On some level we can use them as a reminder of the great diversity of organisms that share the world as our home. A lot of them are cuddly too, which helps.