They say the brain is responsible for our feelings and emotions, decisions, choices… Essentially it is not the heart, but rather the brain, that falls in love and feels attraction to others. Don’t believe it? Mess around with the brain chemistry and see just how quickly a person’s mood and feelings change.
This Walking Dead episode illustrates my point: http://www.youtube.com/watch?v=TX47zIJBxCs&feature=related… He is explaining that every synapse is a memory, a moment, a decision, a fragment of what makes a human being. When that “light” as he calls it shuts off (and in this case is taken over by the zombie-esque virus) then the “human” part of the being is gone, leaving only organic and animalistic drives.