What a peculiar outlook. Are the PCs *always * the Chosen Ones? Is it always the End Times?
That's a surprisingly deep and multi-faceted question.
If we step back from the game for a moment, and consider the people sitting at the table, then there's a general expectation that they'll be playing through an interesting or note-worthy scenario. The fact that the outcome of the game matters to the over-all narrative of the world, is why the players are playing
these characters rather than some
other characters. They aren't necessarily Chosen by Destiny or anything, but they are the ones who are in the right place at the right time with the right capabilities and inclinations to do something. If nothing noteworthy happens as a result of their actions either way, or if Gandalf is waiting off-screen to rush in and solve everything regardless of what they do, then there wouldn't be much point in playing.
Failure on their part does not
necessarily mean the end of the world. However. PCs are fantastically powerful individuals, capable of amazing feats of magic or martial prowess. Maybe not to begin with, but eventually, if they don't die before they get there. A fighter can defeat a titan in melee combat, the cleric is returning people from the dead on a regular basis, and a typical day for the party involves slaying multiple ancient dragons. In order for the scenario surrounding these supermen to be interesting enough for us to bother playing out, they need to face commensurate opposition for the outcome to remain uncertain. And that sort of thing tends to involve the end of the world, or worse.
(This second point may not apply to low-level one-shots or short campaigns, of course, but the first point should still hold. If you're talking about a low-level game, then replace the end of the world with a young dragon which might destroy a nearby town.)
That's out-of-game stuff, though, so your character can't take any of that into consideration when they make their decisions. In-character, your fighter just knows that there is a dragon, and that innocent people are more likely to die if he does nothing than if he risks his own life to stop it; and that choosing to not stop the dragon, and to not even make an attempt at it, is tantamount to abandoning them to their fate. And that
should be sufficient motivation to call him to action, assuming he is the type of individual who is worth focusing a D&D game around.
