I think one factor is that disagreement is more specific.
If I disagree with something you write, I probably want to explain why I disagree: by citing out a fact, criticizing an argument, pointing out a mistaken assumption, or so on. That is, I have actual information to convey.
However, if I agree, then I don't have any specific information to mention. If I say "I agree" I just sound like a dittohead or someone so self-important that I think my opinion is terribly weighty.
I think it's easier to make a nasty comment than an nicer one. And there's usually a call-to-arms to spark a negative comment. Add to that the safety of the relative anonymousness of the web and someone can vent with seemingly no negative consequences other than pissing off someone they don't really know. Much like many actors enjoy playing villains because it's more fun, there's a certain charge to writing something negative. I absolutely LOVE a well written jab or sarcastic comment, but unfortunately most of them aren't well written. There's a maturity issue where a lot of people are (hopefully) getting it out their system. Online widely diverse age groups interact much more than in the real world, so you're exposed to a lot more ass hattery.
If you want to see nice comments, go to flickr - it's a love fest over there.
The disagreement theory is a good one, I think that explains a lot - but it doesn't explain those run-of-the-mill haters who just roam around waiting to personally attack a poster for no apparent reason. I guess that's where the "I'm anonymous on teh intawebs" bit comes in. Fucking annoying anyway.
Also, as Slack has said, most people are more brave when they are anonymous.
I tend to be a tad more forward on the internet than I am in person. Just a tad. I could compose an answer on AskMe that I wouldn't be able to dish out in such a forward manner in real life.
What I don't understand is how people can be so cruel. The homophobia, the insults. Where does that come from? Are all of these comments coming from genuinely hateful people? I have to think yes. This segment on NPR was an eye opener. More than half of the comments I read on YouTube and my local online newspaper are racist, homophobic, or just plain stupid. I guess I'm really naive but I expected the majority of college kids to be above such behavior. Anonymity breeds stupidity.
Same reason people are more likely to complain (to management or friends) about poor service somewhere than to openly praise good service.
People who are pissed are more likely to say something.
But on the other side... I think in a way the internet is making it more acceptable to be, well, a shitty person. There has, and most likely will, always be those assholes who judge someone on what they are rather than who they are, on factors like race, gender, and sexuality. I think in many ways the strides we've made have been in large part because it is now socially unacceptable to do such things. People may think them, but they know if they drop an n-bomb or call someone by a homophobic term they will be more likely to be met with dirty looks from the other tables at the restaurant and what have you. The anonymity of the internet means they can be as big a douchebag as they want without someone really taking them to task for it.
I don't know the reason, but I know it really bothers me. It's the reason I left the Meef. I dunno, the older I get the less I see things in black and white, but I guess a lot of people out there still do.
The incessant namecalling bugs me the most. I would like to think that it is mostly kids, but I know that is not the case. I left MeFi somewhat because of the tendency to attack the poster rather than the opinion. (The main reason though is the lack of nested threads. Trying to follow a discussion there is a headache in long threads.)
i think some of it comes from people's inability to express themselves in text and words in general, so things tend to come across as whiny or defensive when you can detect a tone at all that isn't over the top.
i find it as interesting how often people apologize for comments that are perfectly fine as coming off in some unwanted fashion as i find boring the people who cannot understand what they said as offensive.
I think eth touches on some truth too, when I'm trying to write "funny complaining" - you know like telling a story of how the whole day was shot to hell from the moment you woke up and stepped in a cat piss puddle - it so easily comes off as "whiny bitch" if you don't make the jokes really over the top. If you were telling the story with big smile, overactive handwaving and added drama poses people would laugh though. I've been misunderstood in textreally often as I'm kind of subtle (apparently) but seldom really mad.
I've been hanging around "swedish blogosphere" as of late, or papers with commenting abilities and holy shit to people attack and bitch in these comments! These are trade papers too, seems that everyone who works in advertising is a juvenile little prick with a need to be noticed (jeez, I'm surprised? heh) - it's ten times worse than youtube commenting and what really bugs me is the stupidity of all the comments. You can hear the echo in peoples heads. That's not limited to the comments either, most of the blog postings are really dumb too, where people go on a mile-long hate rant proving that they didn't even read the newspaper article they are linking too - they just read the headline, misunderstood it, and then went off. It's fucking scary.