Look at it like this, if you went for max survivability/mitigation, you would have little room to increase your damage/threat, leading to you not being able to hold aggro on anything. Likewise, if you try to maximize your dps/threat to the point where you become a super aggro magnet but pay little attention to your survivability, you wont be up for long and you can hold aggro when your dead.
Try to keep a even balance of each. Trying to max out 1 aspect of tanking just isnt the best idea. All things in moderation.
"Ive always wondered what we warriors say to make people attack harder, be healthier, or taunt mobs" -Orla