Its because tanks end up running way more dungeons than DPS or healers. As a result, they've done the content so much that they're bored to death of it. They still want the resources of running the dungeon, but they want those resources as fast as possible. There is nothing wrong with that, but if they want to run things fast they need to form a group with that agreement in mind. I play heals primarily and I really hate running WP and AK at this point. I just want to get them done, so if a group takes a while to get through the dungeon, its a coin toss just to see if I'll snap depending on my current stress level.