[Jacob:] Pain came in easily managed forms. The pain of hunger. The pain of cold ice under your paws… Each pain had a simple answer, a clear action to end that pain." (8.211)
Jacob believes that physical pain is easier to deal with than emotional pain, because to end it requires a clear, physical action. Why doesn't emotional pain work the same way?
[Jacob:] "My fault," Edward whispered, and his knees gave out. He crumpled in front of me, vulnerable, the easiest target you could imagine. But I felt as cold as snow – there was no fire in me. (9.142)
In the face of Edward's pain and helplessness, Jacob's rage and hatred disappears. He empathizes with him so much that he feels cold – like a vampire.