The companion is not the audience surrogate

I've seen this being said so many times, and i feel like no one really questions it, just parroting it on to the next person. But we aren't really like the companion in most iterations. Sure, the companion often represents certain parts of us (eg Rose and Donna trying to escape normal life) but honestly we're a lot more like The Doctor when it comes to our worldview.

The Doctor picks up companions to see the world through their eyes, because everything is so new to them. To the Doctor, all of the sights in the universe seem almost pedestrian, whereas the Companion always has a measurable reaction to these strange new things. I think how the Doctor treats them is much the same as us watching them in the episode. At the end of the day we know we're watching a show, so there's a small part of us that doesn't take the show seriously.

The companion is there as a foil to us, someone who takes every moral decision, every strange quirk of the universe, every threat to their life as completely real and important. They keep the doctor in check, who sometimes loses their grip on these things. When the companion says something like "having fun are you?" Whilst they're both standing over a dead body, it's us that are reminded that to the people in the story are real to each other, and in that moment we see ourselves most fully in the Doctor.