In the The Dark Knight, why did Harvey Dent change himself to Two-Face?
Was it because his face was burnt or his love died?
Why he chose to name himself two face seems relatively straight forward, after the injuries sustained (predominately to his face) during the explosion and subsequent conversation with Commissioner Gordon in which it is revealed that members of the major crimes unit were calling him "Two-Face".
Harvey is obviously upset that Batman chose to save his life rather than Rachel's, in addition to whatever emotion he's currently going through to deal with that loss. He sees Batman as the person that resulted in Rachel's death, even though it was the Joker that set up the bomb.
While these factors both play a major part in the beginnings of the psychological transformation of Harvey Dent into Two Face, there is a later visit from Joker which leads him to the conclusion that his frequent personality quirk of tossing a coin to determine his actions could be applied to more sinister and direct action - starting with whether the Joker should live or die.