Mayhaps because you only do casual content where half the tanks never turn off Tank stance and do crap damage as a result? Good tanks, on the other hand, require DPS to use their aggro tools. And if they don't, I will let you die. It's a great teaching method!
I also get commendations for being a "toxic DPS 95% of the fight" healer. Suppose I should keep doing that then. :D
