From Tumblr: Anon Ask - At what point did the Tenth Doctor break?

[nym_wibbly]

Which episode was it do you think where the 10th Doctor really "broke"? Some people write that Doomsday destroyed him and others argue it was Last of the Timelords or Waters of Mars when he lost the plot.
~ Anonymous

In his case I thnk it was more “death by a thousand cuts”. He never got over being separated from Rose in Doomsday, that’s for sure. After that it’s a long succession of incidents and revelations that bring him to a breaking point in The Waters of Mars (at which point the show, with David Tennant leaving, was in the position of being able to write stories for Ten which didn’t “reset” everything fairly comfortably before the credits rolled. It was the only time they could’ve taken the character that far, and they took it all the way when he picked up the gun in The End of Time.)

He heard Donna when she warned him that he sometimes needs someone with him, to stop him going too far. That’s always stayed with me; that he actually needed to hear that said out loud, and that it came from someone who’d only just met him.

42 shook him up badly—I don’t know if we’ve seen the Doctor that frightened before or since. Maybe Midnight. He could’ve reached out to Martha afterwards for support, but he didn’t, and that set the pattern for the rest of his time as Ten. “I’m fine. Moving on.” The events of the final three eps of series 3, culminating with the Master choosing death and leaving him as the last of the Time Lords all over again, were unrelentingly brutal; then Martha leaves him (and he knows he played his part in setting up that outcome, never quite letting her in and refusing to deal with her feelings for him). He reaches out to Astrid almost straight away, connecting with her the moment they meet, only to see her die saving everyone else.

Donna sorts him out and calls him out on the “I’m fine” crap, giving him support even when he rejects it, but that series took the Doctor further towards losing the plot. The Fires of Pompeii forced him to confront what he really stands for (and to personally trigger the eruption that destroyed the town). The Doctor’s Daughter took him to a terrible place, opening up emotionally only to lose someone, again. His encounter with River Song in the Library rattled him good and hard and then she, too, died while he watched helplessly. We get a first glimpse of him being willing to play god when he “saves her” to Cal’s dream world (and later discover that he never once makes contact with saved!River there, even once he understands her importance in his life).

That season sets up the reunion with Rose (et al) and the showdown with Davros, where Davros makes him acknowledge how he depends on the sacrifices (and sometimes the violence/willingness to kill) of others to achieve the things he does; makes him see how he takes people and changes them, and that as much as they adore him and adapt themselves for him, it’s not necessarily for the better. Then there’s two of him and the other one casually commits genocide; then Donna’s his ultimate perfect companion; then he leaves Rose and his half-human self together and walks away while they suck face; then he has to save Donna by erasing the life she loved and making it impossible for him ever to see her again. It’s like Tenth Doctor whack-a-mole, Journey’s End. I think he came out of it pretty broken.

Each of the specials puts another nail in the coffin. He’s travelling alone, and even “The Next Doctor” has a devoted companion. In Planet of the Dead, he rejects Christina’s request to travel with him because he prefers to be on his own, even though they make a perfect team. Then The Waters of Mars. He knows what he should do and he knows what he can do, and he makes the wrong choice because it’s the straw that breaks the camel’s back.

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This post originally contained several reaction gifs of relevant scenes from Doctor Who, a Tumblr thing that doesn't work so well here