Someone is going outside now, and (s)he might be some time.
Setting aside for a moment the one-shot Companions, Adric is unique among the "Children of Time" (as I will probably forever think of them, now*). And I don't just mean that in the obvious sense - true, he is the only one who's life has actually ended, that's it, no more...but, more importantly and uniquely, he's the only one who hasn't figuratively survived the threshing process**.
I had always assumed that, in the grand tradition of Companions, Adric would gradually grow as a person, learning moral lessons from The Doctor just as he leaned cultural and scientific lessons. Actually, before I met the kid, I expected a quieter, more intellectual maths genius - more Zoe, less Jamie (or really Ben is a better comparison, I guess). Once I saw where he started from, though, I imagined the usual sort of growth and enlightenment. By "Castrovalva," I had given up on the slow accumulation idea, too, and was envisioning the final scene of "Earthshock" as a revelation. A final sacrifice to redeem himself, sort of thing:
"Oh, no!" The only way to save the world is to pilot the ship into the Earth! It has to be done manually! Get in the TARDIS, everyone; I'm sending you to Heathrow (I hope) so you'll be safe while I sacrifice myself!" "I'll do it, Doctor; you take care of Nyssa (and Tegan, if you really must)***." "Adric, get in the TARDIS. Now." [Adric locks The Doctor in the TARDIS**** and saves the day Kamikaze-style]
But no, everything was just a mathematical exercise to the end for him. It wasn't necessary for him to sacrifice himself - he just darted out because he couldn't admit defeat on that encryption. I like to think there was some underlying "I need to decode this in order to save the planet," rather than straightforward "I am too proud and stubborn to concede this, to the point of DYING over it," and the homesickness and irritation at the beginning of the serial sort of set him up for a TW S1-S3/29-Jack-esque***** quasi-suicidal world-weariness, I think. Whatever his motive, though, it wasn't really a redemptive ending for him...which makes it all the sadder.
It sort of makes sense, I guess, for the usual techniques to have failed on him, as he is very very much different from any other Companion on a fundamental level. It just saddens me that he never seemed to realize his potential, that he never had a good opportunity to really bond with The Doctor. The only thing that prevents him from being a Special Companion, in my reckoning, is that they weren't close enough. They almost got there in "The Keeper of Traken" (mad props to Johnny Byrne, by the way), but The Doctor was always too busy with Romana or Nyssa or Tegan or The Master to give Adric proper attention any other time. I'm not saying The Doctor failed - he didn't have any choice about how many Children he had to keep track of - but Adric really would have been a better person if he hadn't had so damn many siblings.
And that right there is why I cried FAR more over his death than Four's. Also this:
* Does anyone else find it a bit odd that, of the six Children of Time we have seen Ten with, three of them are played by actors older than Tennant? I mean, Davison wasn't nearly old enough for Sutton, Fielding, or Waterhouse to really be his "children," but they were at least YOUNGER than him. Oh, the completely bizarre world of the Time Lord...
** From a conversation with Captain Chaotica!!:
"So...since all of [the Companions] struck him as having at least _potential_ brains/curiosity about the way the universe works, he tries to cultivate that potential in them. Unfortunately, the Doctor has always believed in TOUGH love--so he does that 'cultivation' with a combine harvester instead of a hoe!"
*** This is Adric speaking here, not me. I quite like her, myself.
**** Yeah, don't ask me how... But it would be a nice reversal of what he did to Susan, I think, so I'm totally going with it.
***** Most convoluted adjective ever? I think so.