This attitude comes primarily from the Death Knight's ability to cause massive amounts of damage with little player effort. Soon the player realizes that he causes far more damage than everyone else & eventually concludes that he is a better player. In reality Death Knights are just over-powered, and everyone knows it.
Death Knight Syndrome at its finest:
One night in a Heroic Naxx raid...
John (Death Knight): Hey, this warlock in blues is *only* doing 2400 dps, can you boot him from the raid or something?
Me (raid leader): He's here on time every week & pays attention. Plus he's a nice guy.
John: Seriously, this free-loader is draggin' us down, we might not get epix... please boot him.
Me: John its fine! stfu & relax, not every class can /faceroll to 4000 dps like you.