How terrible or awesome something that happened to them was. Unremarkable stories just don't get tons of likes/reposts/etc. so stuff gets exaggerated, even though it should be expected that sometimes life is going to be a bit average.

Pretty much everything.

Despite being able to give people access to a plethora of information, the internet is full of highly uneducated and stubborn people. There are many sites with communities that look down on the viewers of mainstream media, because they are susceptible to sensationalism and hate mongering. However, they fall for the same vices when they're apposed onto them. There's been so many threads on Reddit that have started witch hunts over nothing. A lot of people read a title, don't read the article, and don't really look into the reliability of the sources given. An article with a false, but angering, title from an amateur site is good enough to start an internet fight.

For any number of reasons in any combination:

- There are no visual cues in a text medium, so it's impossible to read body language to detect sarcasm.

- Anonymity gives people an opportunity to express themselves without filters or limits they would normally engage when dealing with people directly.

- For the luls, of course. Some people just want to watch the world burn.

- My personal favorite: Poe's Law. There's no escaping it.

Their ego.

Everything!