I've heard it said in places that in the long running TV series Doctor Who the companion is there to give the viewer someone to identify with. However, I do believe that this particular assessment would be best inverted.
We are all the lone expert in our own lives, carrying out our days much like how The Doctor bangs around the TARDIS. Thanks to this expertise, to our companions or to any onlooker for that matter, seeing how we conduct ourselves in our daily lives may be outright mystifying.
For us, our companions are those friends and family whose lives and ours get to intersect for a while. But we ourselves, is the only one who is guaranteed to be always there. The constant. The one that we the viewer can identify with, The Doctor.