What do you think when people post negative comments?

I'm just starting to get into social media online, like watching videos on youtube, reading blogs/ funny stuff and going on facebook/tumblr. I noticed on a lot of these sites people like to post negative and extremely harsh comments. The first thing that always pops into my mind is : "why are are you searching up these shows or videos, posts, ect. if you don't like them?" But then I thought about it and i came to many different reasons on to why somebody might post negative things online. For example one reason may be that; it is the way some people get to vent anger or rage, release some of their built up emotions.

What do you think?