Suave,
A Southern victory on northern soil would have dealt a tremendous blow to Lincoln's administration, which already faced a great deal of criticism from the northern population. Lee understood that perfectly, as well as he understood the fact that if the war were to be protracted, the South had little hope of victory.
So, it was to that end, to strike a mortal blow to the north's will to fight, that he commenced his second invasion of the north. With that aim in mind, retreat without giving battle was not an option. His subsequent, questionable decisions at Gettysburg are predicated upon that fact, and upon the fact that Stuart was joy-riding across the northern terrain, leaving Lee largely without the benefit of reconnaissance of the northern positions and of the strength that faced him.