One of the best things about the web is that it allows people to share ideas across boundaries. Right at this very moment, there could be someone from Saudi Arabia and Peru and Australia reading this article—and as a writer that is immensely cool. However, there's no obligation for all three of those folks to agree. And because the web is not a one-way portal, they're free to share their opinions in response.
Most comments are benign ("Cool idea!" or "This was interesting!") and some are engaging in that they ask questions or share their own learnings on the topic in a friendly, conversational way. Some are less benign but harmless. For example, the person who goes through the trouble of logging into the commenting system to tell you that he wasted his time reading an article or that she was bored by the topic or goes on to list all the ways that you were wrong. But occasionally, someone will read something that they just don't agree with—maybe it challenges her world view—and then all bets are off. Logic and reason seem to disappear. All that matters is that she lets you know how wrong you/the topic/the perspective/the subject is. And what a terrible reflection this is on society overall. Just like that the comment has become a rant. And it will snowball as someone will inevitably try to reason with the commenter or challenge her position or just plain call her names.
This is possible because we can bypass social norms online. We can get angrier online than we might in real the world, and say things that might be politically incorrect because we're cloaked by a degree of anonymity. Although with the growing prevalence of linking social accounts to log-in protocols, this cloak is shrinking. For now, it can be cathartic to speak your mind though these outbursts rarely solve the source of the conflict. While the original commenter may hang around for a few volleys, he or she can depart the site and never return. There is no getting to the root of her discomfiture, no reasoning, and no resolution. There is so self-reflection. Ultimately, you're among strangers. While you may be familiar with some of the personalities behind recurring screen names, if you have not made an attempt to integrate yourself with the retinue of regular commenters there is no obligation to observe the niceties required when relationships exist.
The thing with the web is that we don't have to engage. We can choose to "walk away." Many writers I know have a policy of not responding to comments—it's something I myself do sparingly here on SciAm (though I do read them all) but was thoroughly enjoyed doing on the old home of Anthropology in Practice. But many readers I know routinely skip the comments, particularly for material that touches upon controversial topics. When they do read the comments, their response is typically some variation of "I read the comments. I shouldn't have read the comments. Why did I read the comments?!"
This lament suggests they knew they were going to encounter in the comment thread. People can disagree about everything—from the media coverage surrounding the birth of Prince George to the best type of pets. There are some passionate people online. Still, something compelled them to read the comments. It might be a bit like watching an accident happen: they're powerless to stop it, but won't look away. And once the "crash" has occurred, some bystanders jump in to try to help, while others watch. The motives are simple really: curiosity and entertainment.
Comments help us understand where we fall in the range of perspectives about a particular view. We can agree with the poster (privately) and see how other people treat him when he shares his views. This can give us some indication for how our peers may treat us--unless they share those views. Sharing the same perspective as some of the commenters (whether it's at the original poster or a rebuttal) can help us feel less isolated, particularly if our view is held by the minority. It can encourage us to seek out people who share these views and form isolated groups for support. Or we can disagree with the poster and find comfort in knowing that we belong to some sort of social collective--that our responses are in line with the larger social order.
There is also an entertainment factor to watching people make spectacles of themselves as they doggedly support erroneous facts. It gives us a chance to feel superior. The catch is of course we don't know if the person is serious—whether they actually believe what they're saying or are doing it to get an intentional rise from the audience. So our sense of superiority is short-lived, but it gives a chance to say "I would never do that," even as we shut the tab in our browser.