Now, I know the lefties will chime in with, "You can say exactly the same thing about conservatives." Well, you might, but I don't.
Leftism, and liberalism, and progressivism, and etc-ism. are not merely simple politics for most of these people. Their politics to them are a core part of their identity, and, more importantly, a central support propping up their egos. They are enlightened because they believe these things; someone who does not believe these things, and yet who, superficially at least, appears to be about as smart as they might be, represents a threat to their egos. The foundation upon which a crucial structure of their sense of self-worth is undermined if they discover that there may be people who can pass as normal and intelligent and yet do not believe as they do.
This tends to make the left more emotional and, well, angry when debating issues. It's all well and good to discuss a purely theoretical issue. But when you have a strong emotional investment in it -- when you have skin in the game, as it were -- it becomes not an academic debate but a heated argument. Whether the oil companies should pay windfall taxes is, for me, a theoretical discussion about which I have an opinion but find difficulty becoming emotionally animated about. On the other hand, whether or not someone should be allowed to punch me in the face is a debate about which I might just become a little more heated.
Because there is so much ego involved in the self-definition as progressive, the need for intellectual conformity becomes stultifying. For, if one can believe something different than you -- even if they are, on balance, progressive -- this represents a direct challenge to your own self-valuation as an intellectual worthy. Conservatives have sometimes heated debates about what the right position is, but they're not debates about whether or not someone is worthy or intelligent. Whether or not someone is a "good conservative" is sometimes argued, but that argument is not simultaneously about whether someone is a good human being. Lefitsts, sometimes by their own surprisingly honest self-admission, acknowledge that they are sometimes too eager to define and, worse still, enforce a group-mediated orthodoxy on the wider church, and excommunicate those who are deemed heretical.
Leading lights of leftism have a powerful psychological tool for enforcing their own preferred orthodoxies. It's one thing to tell someone he's wrong on an issue; it's another thing to tell him, impliedly, that he's evil or stupid because of his stance on an issue.
Yep. We conservatives are racists, fascists, hatemongers, homophobes--and we drink the blood of infants and grandmothers.
Why is humor and irony so common on the right and so hard to find on the left? Humor and irony require emotional distance from a subject-- something I would contend the left is in of rather short supply.
Yep. As I said in my bumper sticker post, lefties usually aren't funny. Jon Stewart can be funny, but he's an exception.
Pre-Emptive Rejoinder: Please, Larry the Urbanite, do not tell me you are angry because you "care" so damn much for this country. I care about this country too; not to get into a contest about it, but I'm pretty sure I care more about America itself (as opposed to "America and the rest of the world which is also, in a way, America") more than you do. Your anger cannot be explained as simply "caring" more. I grant you-- you do seem to be more emotionally invested in these debates. But the question remains: Why? Not because you care about America. There must be a more personal reason for the perpetual fury.