Originally Posted by BatLobsterRises
It was made clear in Batman Begins that this version of Batman didn't intend be there forever to beat up every single criminal he could get his hands on, but had the more tangible ambition to "show Gotham that their city doesn't belong to the criminals and the corrupt" aka, get rid of the mob's stranglehold on the city. By the end of TDK, not only is Gotham well on its way to achieving that, but he's destroyed all meaning his symbol had in an effort to make a martyr out of Dent.
Anyway, even if you don't like the Dent Act as a plot device, that doesn't change the fact that it's the reason the writers provided for why he stopped being Batman. It doesn't have to do with Rachel's death. The point is that things like Rachel's death do weigh upon him, but he's stripped of his usual method of dealing with pain- being Batman, which gave him purpose. That is not the same as "I am sad, I quit."
Either way he quit, and then took the fall for Dents terror spree. Why so people would keep "believing in Harvey dent", which is truly heroic on his part, but logically he must of known a man Like Bane would come along and expose the harsh truth.