Can someone explain to me?

The easiest explanation I can think of is the superiority complex, where they believe they know best. When one believes they know best, they try and impress the 'correct' opinion onto others, and feel that they should educate them to the 'correct' facts. They perceive themselves as the correct opinion, because they themselves believe it as fact that they are correct. Others just like the anonymity of the Internet to insult and degrade, and having a place where Du can target your specific dislikes, amplifying what may be a mild dislike into large proportions, and face no repercussions, unlike human interactions. Some people though, are just dicks.

Agreed. In my mind if someone has to insult to prove thier point, it lowers thier percieved intelligence. I will listen to about anything, if it's done with respect.