The motivation without giving too much spoiler is :
act 1) Solve the issue in the first area (conquer or die)
act 2) Increase your power
act 3) Impose your power
(at least it was for the motivation that transpired)
Those are the motivations, not vengeance, redemption or preserving someone/something.
Those are a byproduct of the 3 points above.
And those motivations felt very good at the end of act 2. I was jubilant. (only to cry when I saw what act 3 actions were

)
As 10000hp said, what you do is creating the legend of your character by taking meaningfull choices to get you to power.
And the choices are most of the time far away from the classic good/neutral/not so evil one.
It's more about fair and unfair choices to tell the thruth, which is something so far unique in a RPG.
And I really had the impression that I had impact on the world with character reacting and speaking about what I did and my choices.
So much that after I ended the game during the week-end I started a new game to take another path (and a higher difficulty).
I really wish that the feeling I have is true but I will have to check by playing this new time.