Robert Tracinski has some thoughts on why leftists get so mad and so personal in political disagreements. Although I think he gives conservatives a tad more credit than they deserve, I generally agree with his points. But I think it's not just that their hoped-for Revolution depends on other people--it's that their whole reality depends on other people. They think that the evil in the world is the result of social structures (everything in the world is the result of social structures) that are ultimately within our control if only we'll exercise sufficient will power. Yes we can! sounds inspiring, only until it turns into Why won't you?! And it always does.
For example, crime or discrimination are the result, for the leftist, of institutions, not spontaneous orders, and therefore are caused by somebody's sin. If you don't act to change things, therefore, you're part of the problem. Poverty? We can cure it by raising the minimum wage. Because poverty is caused by greedy people being stingy. And if you oppose us, you're helping cause poverty!
Yet in reality, many of these problems, including poverty, are not caused by design, but are the consequences of nature or spontaneous orders. If you think the laws of economics are not natural laws but the result of human consciousness, then, you're unable to distinguish between those things that can be cured by conscious acts, and those that can't. And this, in turn, affects your thinking about justice. It leads you to think natural things can be unjust--that inequality is per se unjust. If you think that way, you'll soon think you live in a world that is absolutely saturated, through and through, with injustice and unjust people who are basically only making things worse all the time. How could you not go through life furious with indignation?
If you're ignorant about economics, for example, and don't know how prices work as an informational system in a spontaneous order, then you'll think that the reason things are expensive is just because greedy people put big price tags on their stuff. If they weren't greedy, they'd charge less. And that greed makes people poor! It's all the result of evil people doing evil things. If you're ignorant about socio-biology (or just refuse to believe it) then you think the reason women are on average less likely to occupy management positions and earn the same as men is because of evil sexist corporate managers making anti-women choices. If you're ignorant about justice and think inequality or unfairness are the same thing as injustice, then you're going to think that nature itself is unjust, since nature distributes her gifts unequally. If you go through life believing that reality is ultimately about other people instead of being ultimately about your interaction with nature, then you're going to think all the bad stuff is ultimately caused by other people, and that's going to make you hate other people, even while you profess to love humanity in the abstract.
Left political thinking is, largely, ultimately a conspiracy theory. The institution of private property, for example, was decided by exploitative elitist a as a tool for exploiting the workers and monopolizing the social surplus. The rest of capitalism flows from a consciousness rooted in this greedy design, and devoted to perpetuating it. People either know this and are consciously aiding in it, or they're dupes who need to be educated and mobilized. Well, if you think politics is ultimately a conspiracy of the evil Koch Brothers against the People's True Path to equality, then naturally you're going to be angry.
Obviously this doesn't mean that leftists are wrong to think there are a lot of injustices in the world, or that injustices are often caused by evil choices or failure to act. Everyone rightly recognizes that there's plenty of bad stuff out there we could do more to fix. But this basic leftist premise--what Rand called "social metaphysics" or "second-handedness"--this idea that reality is ultimately about people's acts, not about nature--is what makes the difference. It's the reason, ultimately, for the long leftist tradition of exhortation for revolution--as opposed to the right, which typically focuses on the bourgeois virtues of going out and getting a job and pursuing happiness. As Martin Malia writes, there really is no such thing as socialism to compare with capitalism. There's just capitalism versus some vague anti-property rights, anti-free exchange abstraction of the ideal. Your goal as a leftist, then, is ultimately the transformation of all of actual society into something equal and fair, but since this is impossible and even meaningless, it's a perpetual exercise in futility. The leftist, understandably given his basic assumptions, assumes that this failure is caused by lack of faith. Yes we can! so Why haven't we?! Because we're evil, that's why.