A/N: This story was inspired by "The Doctor, the Widow, and the Wardrobe" and contains some spoilers for all of Season 6, plus the Christmas special.
Also, I really, really hope this is dealt with in Season 7, even if it totally blows my story out of the water canon-wise just weeks from now.
I have to give significant credit to madis hartte's brilliant "Why Bow Ties are Cool" for the John Smith backstory. It is completely wonderful, and you must read it if you haven't already. Seriously, go now.
It wasn't as if he hadn't used pseudonyms before.
"John Smith" had once tripped off his lips almost as naturally as "the Doctor" (though he'd used it very little in his current regeneration. After his time in 1914 he'd found himself reluctant to trot out the name, as if the person he had become in those weeks would follow) and he'd never given it a second thought. "The Caretaker" should have just been another of those one-off names that he adopted and discarded without a second thought. It wasn't, though.
The thing about needing everyone, even your closest friends, to believe that you were dead was that it meant everything had to change. No more "Hello, I'm the Doctor" introductions were just the start.
This explained why he'd spent a fair amount of time contemplating which name he should use when Madge and her children finally arrived at the house. "The Manager" and "The Professor" had been up for consideration, as had a variety of human names. He'd quickly eschewed the latter. If he was going to pretend, at least he was going to pretend to be something close to who he actually was.
As a title, "The Doctor," he'd always thought, had been a stroke of genius. Doctors fixed things. He fixed things. They were wise. He had (or at least he had wanted to have in his youth, and flattered himself that he had achieved some measure of in his more advanced age) wisdom. Doctors knew the answers to questions, and they knew the solutions to riddles. In short, they knew, and the joy he had found in the sheer pursuit of knowledge was one of the few things he felt defined him back when he was young and trying to decide on a public identity. So he had become the Doctor.
None of that, on a purely objective level, had changed since his so-called death at Lake Silencio.
What he knew must change, however, was the way he lived his life. No more dramatic rescues he told himself. No more altering the course of history he'd repeated over and over in his head. As he'd said to Dorium, he'd gotten too big, and the natural corollary to that notion was that he needed to be smaller. Act smaller.
Caretakers fixed things. Caretakers could answer questions, though perhaps not always the kind of questions a Doctor could answer. Caretakers could be wise given the correct circumstances.
Caretakers, he decided, were likely brilliant with riddles.
On a fundamental level, Caretakers looked after people, and in a very real sense, looking after people was the thing that the Doctor (after centuries of travel and wisdom-seeking) found more joy in than knowledge itself.
So he was the Caretaker.
In a way (in many ways), becoming "the Caretaker" was incredibly freeing. There were no expectations (his or others') for the Caretaker. He didn't feel the burden of a thousand other occasions weighing down on him. In fact, nothing weighed him down. The Doctor had altered the course of History itself, but the Caretaker? All he wanted to do was create a perfect family Christmas for one grief-stricken woman and her two children. History didn't care about Madge, Lily, and Cyril. The Caretaker did. The Caretaker could.
Sure, things had gone a bit shaky 'round the middle, but as far as adventures went, it was still a relatively small one in the Doctor's long history. For the Caretaker, however, it needed to be an outlier. A rare occasion for excitement and saving the day that must not be repeated else the Caretaker could turn back into the Doctor all too quickly.
He couldn't let that happen.
So after he'd left Madge, after he'd had Christmas with the Ponds, after he'd thanked River in multiple ways for telling Amy and Rory that he wasn't dead (even though the River who had been taking cookies out of the oven when he'd arrived hadn't yet told them anything of the sort), and after he had settled back into his solitary life on the TARDIS, he had decided to just be the Caretaker.
In many ways, it was the most liberating decision he had ever made for himself.
He stopped looking for interesting pockets of time to visit. He told the TARDIS (told in the literal sense, as he knew full well that the voice interface worked perfectly well from his end, though only sporadically and more than a little oddly from hers) to avoid events that would attract notice. He wandered through markets, talked to humble people, and rarely left more of an impact on the lives of those around him than the fleeting memory of a kind face and floppy hair.
Sometimes he would give aid to those who needed it. He helped a family rid their home from an infestation of half-phased probe droids, assisted a young girl in her search for a cure to her brother's mortal illness (the Sisters of the Infinite Schism had been particularly useful on that occasion), and had even retrieved a kitten from a tree one lazy afternoon.
In short, he tried very, very hard to be the Caretaker.
Every time he solved the small puzzle for that day or received appreciation from someone he had assisted, though, he couldn't help but wish for the days when he had been able to fix the really big problems in the universe. Those he helped never knew that a small piece of him regretted the time he spent with them (time he could have spent saving worlds) but he knew.
It should hardly have been surprising, then, that the siren call of adventure (real adventure) would eventually tempt him back into his old life.
Back to the Doctor.