Why won't the Doctor do this?

Question

In Series 9, Episode 6 of Doctor Who, he explains

to Ashildr, "the woman who lived", that he won't travel with immortal people, because he rather travels with, what he calls, mayflies. He says he travelled with an immortal person, Jack Harkness, once, and he won't do it again.

But that's poppycock, of course. He travelled with other long-lived people in the past, both from his own species (Susan, Romana) and humans (I count River, who could regenerate after all), and he never had any qualms about it. So obviously, that's not the real reason. Surely he's holding something back.

So, is there a real reason the Doctor doesn't want to take Ashildr with him as a companion?

What is it?

Answer 1

Ashildr and Harkness have more in common than long lifespans.  Those two each have a type of immortality that was acquired unnaturally and from completely external sources.  Also, they both occur in Nu Who. 

Even if we assume that the young Doctor did choose to travel with immortals like Romana and Susan*, it was a young Doctor making that choice.  Things have changed.  Twelve has lived through horrors that his first seven incarnations had never known.  He's a changed man, and it's not so shocking if we find that his opinions on this matter have also changed.  If this question has an answer, we should be able to find that answer in Nu Who.

River is a special case.  She's the Companion that, so far, has not travelled with the Doctor so much as she's made a habit of summoning him.  The dynamic between these two characters is unique.  I don't think we can learn about how Twelve views Ashildr by looking at River. 

Twelve mentioned Harkness.  Let's look at what Ten has to say on the subject: 

It's not easy even just looking at you, Jack, 'cause you're wrong. 

You are.  I can't help it.  I'm a Time Lord.  It's instinct; it's in my guts.  You're a fixed point in time and space; you're a fact.  That's never meant to happen. 

Harkness was, at first, an ordinary human being.  Along the way, he lost the ability to die.  In effect, an integral part of his nature was stolen from him.  Ten can't help but see him as an abhorrent monstrosity.  He is as disgusting to the Doctor as a zombie might be to you or me. 

Ashildr's immortality suffers from the same weaknesses.  It's just as unnatural.  It's something that was done to her.  Even more relevant, it was something that was done by the Doctor's own hand. 

I think you're right.  It's not simply immortality that the Doctor can't stand.  We've seen Ten and Eleven wish for the companionship of fellow Time Lords and other long-lived creatures.  The immense life spans of creatures that normally have such life spans don't seem to bother the Doctor at all. 
 

Leaving Ashildr behind is just another example of the Doctor running away from a mistake that he can't fix.  He can't stand travelling with her because she's the wrong kind of immortal.  Taking her with him would mean that he'd have to face the mistake he made, day after day, without being able to make anything better.  If anything, his company would make the mistake grow worse and worse. 

And, maybe, her having human companions might make her better.  After all, that's a technique that's worked for the Doctor himself, hasn't it? 

_______________ 

* I'm not even sure that Susan was immortal.  Oh, we can claim that Time Lords are immortal and we can claim that Susan was Gallefreyan.  I don't think we can claim that all Gallefreyans are as immortal as the Time Lords.

Answer 2

The Doctor does lie, of course, constantly. But that doesn't necessarily mean we know the truth. That said, I think we can mostly take him at face value.

• When Susan traveled with him, we actually had no idea what her, or his, lifespan would be. There was, in fact, no reason at that point not to believe they were human beings from the future, rather than aliens who happened to look like us. Also, as hinted at not only this season but in the past, we don't really know why the Doctor left Gallifrey. Therefore, we don't know what extenuating circumstances may have driven him to feel like he had to take Susan with him!

• Romana was foisted upon him by the White Guardian as part of the Quest for the Key to Time. She then chose not to return immediately, having grown fond of the traveling life and recognizing how stifling her life as on Gallifrey. She was young, for a Time Lord, when she met the Doctor -- 140 if I recall correctly -- and naive, and provided him many of the same "foil" characteristics his usual human companions provided, as she had not yet lived long enough to grow jaded.

• River Song never traveled with him as a companion, and for many of the Doctor's encounters with her, he had no idea she wasn't an ordinary human being. He occasionally stole her away from prison for an adventure. That said, River was not actually all that long-lived, having forfeited most of her regeneration energy to save the Doctor. At the point she did so, she was maybe 40 linear years old. Based on "The Husbands of River Song", and assuming she wasn't lying when she claimed to be around 200 years old then, we know that River Song ultimately lived to be about 224 linear years before dying in the Library. (Since she never looked significantly older, we can surmise that she was physically aging much like any Time Lord incarnation would.)

By the time the Doctor re-encounters Ashildr, she's 800 years old -- older than the Doctor himself was through most of the Classic Series. She's clearly quite jaded, and suffering from the detachment that comes naturally from such a long life, coupled with an ordinary human memory that easily forgets. In short, she has almost all of the characteristics that drive many fans really nuts about particularly the Twelfth Doctor, but she's had no-one like Clara to kick her in the behind when she starts getting too jaded. The Doctor can't provide that service for her, so even if he did take her along, they'd still both need someone like Clara to keep them grounded.

All in all, I think the Doctor's explanation stands up at face value, just fine.