Character ambivalence + conflict

I've always admired films in which the ambivalence of the protagonist creates the main conflict in the film. Ambivalence in this case would be defined as a character wanting two things simultaneously - or not being sure what he or she wants most.

A great example is On the Waterfront, in which Brando's conflict arises from his love for Edie and his loyalty to his brother and the union. Without this conflict there would be no story to tell. And it's his resolution of that internal conflict that leads to the climax of the film and his inevitable character change.

Your thoughts, other examples?