[This is, I suppose, a sort of theme post for .hack. It is not tied to any specific episode.]
Tsukasa asked, “Why me?” (In the dub, not the subtitles. Just go with me here.)
That question is somewhat difficult to answer. On one level, it had to be someone, and given a finite number of people someone has to be the best candidate. It could be that there's more than one such someone, but there has to be at least one person whom no one is a better choice than. It's just math.
On the other hand, of more than 20million people, is Tsukasa really the most... well, most what? What are the selection criteria?
My short answer is that's he's depressed, but I think there's a lot more to it than that.
It's often the case that protagonists are almost blank slates when they start a story. Consider Harry Potter and Bella Swan. When their stories open they've got no friends, no connections, nothing tying them to the world they've known. Bella's major discomfort in picking up everything and moving to Forks is that she's moving to Forks. There's nothing and no one in Phoenix that she doesn't want to leave behind. When Harry heads to Hogwarts he doesn't have anything that he's going to miss from his life before.
This simplifies storytelling, Bella doesn't have any friends from back home to stay in contact with, Harry isn't trying to explain that, no, he hasn't joined a cult, to his friend he's not allowed to reveal the wizarding world to. Neither of them is juggling old commitments with new. Neither of them is having longing for what was given up drag on their new shiny sparkly life.
But they could. Bella could have had friends. Harry could have had friends. Phoenix/Muggle friends would have changed the stories, but they wouldn't have invalidated them. Bella still could have snagged a jerkish vampire, Harry still could have been a wizard. They could have actually had lived in lives before they started their respective stories and still had their stories come to pass. Tsukasa couldn't.
It is my belief that if Tsukasa had had friends when the story started then Tsukasa wouldn't have been chosen. Someone else would have. Someone more like Bella and Harry in that respect.
If he had friends online then he'd have someone to turn to when things went wrong. Also, even if he'd never said where he lived it's possible that their past interactions would have given some indication. There would be a definite possibility for someone tracking down real world Tsukasa based on that. Without that tracking down the body outside of the game becomes a much more difficult proposition. Definitely not impossible, as we shall see, but significantly more improbable.
If she had friends offline then there would be something to go back to. Ditching the real world and accepting that being trapped in the game wasn't a problem likely wouldn't happen, or at least not so easily. Also she might have invited one or more them to play the game with her. That would have changed everything in a way I don't think DVL* would have allowed.
Tsukasa has absolutely no one passing him information from the outside world, he doesn't even know that his body is in a coma. Hell, for all he knows he's been transported into the game body and soul. If he'd had someone telling him, “Hey I just visited your body in the hospital and you really don't look so good,” that might have given him reason to want to get back to the real world because it would indicate that his absence is harmful.
More than that I think one of the most important things in choosing Tsukasa was that Tsukasa had nothing positive to build on. The only indication we ever have of someone taking notice of the Tsukasa in the game before the show starts is when he was seen with Macha, and in that case it was Macha, being a conspicuous anthropomorphic cat in a pointy hat, that attracted the attention. For everyone else there's a prior history (though in Mimiru's case you have to wait until a bonus episode to learn about it) friends that they've had, connections they've made. Tsukasa has nothing.
The only time Tsukasa has anything approaching a happy memory (from on or offline) is when he's reminded of movies his mother liked, and that's quickly overpowered by the fact that memories tied to his mother make him sad as she's long dead.
He has nothing to look back to. He doesn't have friends, he doesn't have connections, he doesn't have joy. When things get bad he can't go to his happy place because he doesn't have a happy place. I think that's why he was chosen.
That he's susceptible to depression and has a lot of painful memories to draw on is certainly a part of it, but I think it's the lack of good more than than presence of bad that made Tsukasa be the one picked rather than the millions of other options. Others amoung those millions have probably had to face things worse than Tsukasa ever approached**, but Tsukasa has just the right mix of trauma, sadness, and a lack of positive things to make him the perfect candidate.
Because he's lived a pretty well joyless life up to this point, any bit of happiness that Tsukasa has, any good thing in his life, is going to be built up in The World after the series starts. That means that it can be destroyed in The World. It means that if you control the game you control Tsukasa's emotional well being. He has no friends, he has no positive history, he has no happy place to go to when times are rough. He has no tether.
That's my best guess. It definitely wasn't an accident that he was picked. By the end you'll see indication that this wasn't random, he was singled out. But you never get the full thought process behind it. You can only speculate, and this is my speculation.
He's completely on his own with nothing to hold him steady. That makes him an ideal person to manipulate. Control the game and it would be the easiest thing in the world to build him up and then send him into a shattering fall. But you can't control the game. The game is full of people, and people are not pawns that can be moved about the board as one pleases.
DVL should know that better than most, she was never supposed to be evil. She didn't fall into line, and neither will those she attempts to force into her desired narrative. That's where the story happens.
* Disembodied Voice Lady. Try to get used to the abbreviation because I will be sticking with it.
** That said, I don't want to minimize what Tsukasa has had to face. Tsukasa has faced the loss of a family member, abuse (including but not limited to physical abuse) from the parent who remained, is in an incredibly unpleasant and fragile place emotionally, and I've just spent five posts talking about an episode where his most consistent reaction was to assume that any newly arrived person on the scene was going to hurt him.
I may resist concluding that Tsukasa has had to face the single worst life of everyone who has logged onto The World, but I want to be very clear that Tsukasa has had a very bad life. Worse than anyone should ever have to bear. When approaching the question of why him instead of someone else it makes a certain amount of sense to try to find something approaching an objective standard for who has it worst, in any other situation it does not.
Tsukasa has had to face worse than anyone should ever have to face and in almost every conceivable context it is entirely unhelpful to respond to that by asking, “Does anyone else have it worse?” As someone recently pointed out, the worst thing that's ever happened to you is the worst thing that's ever happened to you regardless of what may or may not have happened to someone else.