To my mind, a purely altruistic, goody-two-shoes hero is even more boring and unrealistic than a purely evil villain. Maybe because at least the villain is getting something out of being a villain? All those armies to order around, and castles, and power, and so on.
OK, people do wake up in the morning and think “Today, I am going to be Good, because it is the Right Thing To Do!” Or at least, we want to think there’s somebody out there who does. So the uncomplicated Good Guy is a little more believable than the Bad Guy who has no motivation other than lust for power.
But purely altruistic motives are rarely strong enough, all on their own, to drive a hero through the pirana-filled moat, across the blazing courtyard, up the wall covered with razor-sharp poisoned spikes (and greased), and past the evil dragon to retrieve the lost magical whosis. Of course, it’s quite possible for the hero’s motivation to be a deep-seated desire to be looked up to, or an inability to live with himself if he fails…but as soon as you give him those, he’s not a pure altruist anymore, is he?
And even people with “pure” altruistic motives often have reasons behind their reasons — they work themselves to death to feed the hungry because as a child, they had to watch their little brother starve to death and they’re trying to make up for it, or because they get an emotional rush from having the power to save lives.
Very few of the fantasy heroes I can think of really do have purely altruistic motives. They’re usually subjects of the threatened king or citizens of the threatened kingdom; their families and friends are at risk if they fail in their mission. The real problem is that the authors often don’t think the heroes’ motivations through, in the same way they don’t think their medieval background through — they just assume that the story will be set in generic Fantasyland, and that the heroes will have generic we-gotta-save-the-world motives.
I have had the problem that, in the course of trying to understand how it came about that my heroes were going off together on something resembling the generic fantasy quest, I realised that it was because they were all, in their various different ways, fairly screwed up, crazy or deluded. I did wonder if they were really doing the right thing, but the villain turned out to be a bit wacko too, or at least he had a very odd world view.
Altruism and fatalism (accepting their appointed task) can be hard to distinguish sometimes. Was Gandalf primarily altruistic, or was he simply constructed so that carrying out his assigned task was his top priority? (He seems to be a constructed being roughly on a par with an “angel”, made for a purpose.)
For that matter, what WAS Frodo’s motivation? We know why Sam and Merry and Pippin were involved, and Aragorn, pretty clearly.
DDB-It’s pretty clear that Gandalf could have chosen to do other things (after all, Sauruman did, and he was the same sort of constructed being). So he may have started off with “saving the world” as his particular assignment, but at some point he made an active decision to keep doing it for his own reasons as well. That makes him a more complex character, as far as I’m concerned, even if we never do really find out what his own reasons were.
Frodo…I’m not sure about Frodo. At the start, he’s just trying to protect his home by passing the Ring on to somebody wiser who can “handle it.” After Rivendell, he’s kind of stuck with it. But I think the fact that you can argue (a lot!) about exactly why he volunteered at Rivendell means that he’s not clearly and obviously a purely altruistic character, either.
And of course, all of them change greatly over the course of the story, and their reasons for doing things change (or they collect new reasons, which isn’t quite the same thing, just close – like Merry and Pippin ending up in service to Theodin and the Steward). If your primary reason for doing things is “It Is The Right Thing To Do,” that doesn’t usually change, or need to change, over the course of a story (though exactly what the Right Thing To Do is may very well change more than once).