Mainly because your Warden's goal is to start up the Wardens again (since everyone died in Origins...except you, Alistair and/or Loghain). But of course gets sidetracked with MORE INTELLIGENT DARKSPAWN!!!!!
"Join us, brothers and sisters. Join us in the shadows where we stand vigilant. Join us as we carry the duty that can not be forsworn. And should you perish, know that your sacrifice will not be forgotten. And that one day we shall join you.”