Rose:
Rose is really young. One understands this immediately, but the show is meant to appeal to kids too etcetc, so it doesn't become salient until the examples of Martha (in her mid-20s) and Donna (early-30s, I think?). Of course, everyone is young compared to the Doctor. But neither is the Doctor necessarily older, in the sense of more mature. XD; So far so good: but, he could be. There's a fluidity, a circumstantiality to the character that I'm only starting to grok, that goes beyond the obvious Doylesque fact that he's played by a succession of actors into the realm of the conceptual... or does it? (Old-skoolers welcome to weigh in here.)
IMO Rose treated Mickey very poorly, but in a way that's easy to comprehend - personally, I can't look myself in the mirror and say that I would've been more thoughtful and clear-headed about it when I was 19. What was she supposed to have done, anyway, break up with him? The first time she came back and it had been days for her but a year for them? She didn't stop caring about him, precisely. It wasn't like that with the Doctor. Only there was this bright poison that rendered everything else pallid, and she'd never been passionately in love, either, not with a person nor with an idea: not to the extent of having a basis for analogy. If she'd known herself better it would only have made her more ruthless. She was really young. Donna's priorities shifted as well, but she was old enough to understand what she wanted every step of the way.
(This is the sort of emotional note Davies knows how to strike - QaF was nothing but. There's other stuff he doesn't quite have the ear for. XD;)
IDK what the going school of thought is on the regeneration process but it doesn't make sense to me that it would be 100% random. It's too easy to posit that Nine's caustic leather-clad loner pose came about as a result of the War (the things he says about humans, when compared to Ten!). In which case Ten is in some respects a response to circumstances as well; but an odd one, if he's a response to Rose. There's a tension to the Nine/Rose trajectory, discovery and friction and testing of boundaries even as their rapport is immediate (not to mention Jack gets thrown in the TARDIS dynamic), and it all but evaporates in S2. You could say they've gotten comfortable with each other, but it's more than that. Sometimes they act like no one else in the room is entirely real; as if they're encased in a bubble and all the pain in the world awaits outside... I found it frequently irritating. XD; Ever tried watching a self-elected high school cool kids clique of two go about their business. That vibe of teenage innocence on top of it all - a younger teen than Rose actually is - all the intensity massed just this side of sexual. It's tempting to say that about Ten, that he's really young too, but he's not. He was different with Rose. Not that he forgot anything, but some difficult-to-quantify parts of him got packed up and put to a corner, and that allowed him to... who knows. Pat extrapolations aside. I don't have the data to say Rose was the Companion the Doctor loved best (wtf does that even mean in his context anyway), but I find it hard to imagine that he would've had that flavour of relationship with many others. Due as much to context as anything else.
Girls Aloud - Untouchable (Radio Edit) This may say more about me but the word that keeps coming to mind re: the Ten/Rose attention bubble is "dangerous". XD;; I mean, it does go pear-shaped and "Doomsday" is horribly tragic precisely to the degree that they're so intense about each other. But if not: Rose quite meant it when she said "Forever," and the Doctor didn't doubt her. She's one for making the universe bend to her will, and Jackie's motherly premonition was chilling because correct. Eventually she wouldn't have been exactly Rose, but then the Doctor would no longer be exactly the Doctor... right? Incidentally I watched some more of the Confidentials, including end S3 where one or other of the producers explained that the Doctor and the Master had... gone to Hogwarts school together... =_= I was hashing out the thought process detailed above and thought Oh gawd, like a sort of Marauders-era!Doctor. Which without S2 would have been a mental leap, actually.