The Exile forms bonds with people (like Bastilla and Revan) and when they feel pain he feels it as well. During the war when so many Jedi that he formed strong bonds with were dieing he felt the pain (like in the beginning when Kreia has her hand cut off) so he cut himself off from the force (yes he did it to himself, atleast thats what I got from it). That is why Kreia likes The Exile so much, she hates the force and she loves his ability to 'kill' it...