Tobold has an interesting post, Making Quests Less Anti-Social, where he argues that people do not group up for quests because quests "*must* be done alone if the players want to maximize rewards".
I agree that most people don't group up with strangers when doing solo quests. However, I've always found grouping to be more efficient, especially for reducing downtime. Additionally, grouping with people is usually more fun than going it alone.
Second, if players were going solo to maximize reward, I would think that if you asked someone to group, they would turn you down. Yet my experience is that if you encounter someone doing the same quest as you, and you ask if you can join their group, 90% of the time they will send you an invite. To me, that behaviour implies that maximizing reward is not the reason that people don't group.
I think people don't group because they are ambivalent about approaching strangers. Maybe it's fear of rejection, a desire not to impose on someone else, or feeling bad about asking for help. But my experience is that a lot of people are perfectly willing to group up, they just don't want to be the one to ask. And because you can solo most quests, they don't ask unless they have to.
In some ways, I think this behaviour is at the heart of the whole 'clique' issue in a lot of guilds. You join a guild, and your guildmates become something more than strangers, but less than friends (at least at the start). It's still hard to approach them and ask if you can join them, because they are sort-of strangers. Yet it still stings when they leave you out, because they are sort-of friends, and you expect your friends to ask you to do stuff.
So I don't know how to solve this. Maybe Public Quests in Warhammer Online will solve this problem, by implicitly grouping people in the same area on the same quest, without making one party formally ask and risk rejection. In WoW, though, if you are willing to take the first step, and ask for help on the General channel, or whisper someone you see working on the same quest, you may be surprised at how easy it is to group up with a perfect stranger.
Funny/weird grouping story: A couple of nights ago, I grouped with a mage to do another Arathi Highlands quest. This mage didn't like buffs. He didn't run Arcane Intellect, didn't cast a mage Armor. He even asked me to turn off my Aura (I was on a paladin alt--yeah, I'm not really sure why, either). I wasn't able to figure out why he had an aversion to buffs, but he was a nice guy in all other respects and we finished that quest easily.