Note that so could the player, and that that no solution from the player was forthcoming.
As a player, I would feel like I'm overstepping my bounds if I tell the DM what his NPC's or what his world is. I tell the DM who my character is. It's in his hands to get that character where he needs them.
If I say "My character is too poor to get a plane ticket across a continent," I wouldn't expect that to be a barrier to entry. I would expect the DM to gloss it over with some justification.
But there does come a point where the DM simply doesn't have to time to keep brainstorming up reasons for a single player to participate in the the game
I'd argue that this job for a DM is more important than almost any other job, so if he doesn't have time, he better make time. If you can't properly motivate a character, and if you can't make the adventure a narrative convenience, you aren't doing your job as the facilitator of the game.
That's not to say that there aren't disruptive players out there, but that is to say that the DM has a lot of control in how disruptive these things can become. Putting a cap on it and getting on with the fun part should be pretty easy.
I disagree more-or-less completely. You're playing adventurers, go adventure. Why shouldn't the players meet the DM at least half way?
If your adventure can't come get them, why would they want to do it?
"Adventurer" isn't everything everyone plays, even in D&D, let alone in the broader scope of RPGs. Even within the scope of "adventurer," there are different motives, different styles. Some will quest for glory, some for gold, some for justice, some out of necessity. Harry Potter just wanted to escape his awful home life, he never wanted to be the Chosen One. Bilbo Baggins was cajoled and taunted to go on his little quest. Conan saught conquest. Samwise was just following a friend.
The D&D-style "career adventurer," or the D&D-style "fantasy mercenary," are valid archetypes, but they shouldn't be the only valid archetypes, even in D&D. If I give a group of paladins and clerics a quest whose only reward is a fat sack of cash, I shouldn't really expect them to take it. If I give a group of thieves and warlocks a quest whose reward is saving some servant-girl, I shouldn't really expect them to take it. That's why multiple quests, many paths, and no bottlenecks, can be so important.
Why should the DM have to? It's not only his job.
It pretty much is: get PC's into adventures. If you can't do that, and if you can't extend a little bit of effort to do that, you're not doing your job that well.
This I agree with more-or-less completely. No, make that completely.
That idea -- "Tell me why your PC is in the dungeon" -- gives the DM a lot more work than preparing hooks, though. Because if your party of five characters has five different reasons for being in the dungeon, they all have to be plausible. One person wants a sack of gold, so that's in. One person wants ancient tomes of knowledge, so that's in. One person wants to save the princess, so that's in. One person wants to discover more about their family's old properties, so that's in. One person wants to unleash the forgotten evil that lies at the core, so that's in. It mandates what you can have in your dungeon.
I, personally, like playing that way, but I'm a huge fan of improvising. Most DMs have a dungeon that already has monsters and treasure. What these DMs need to do is figure out a reason for the characters to visit it. If your reason doesn't work, try a different one.