Why does Death feel serenity when fights him? How is this ironic?

Death remarks that it actually visited the room when Liesel was absent and prepared to take Max's soul, but felt a struggle and withdrew. Death feels a serenity because he so often takes souls who are withered, worn and content to die. Death takes no particular pleasure in taking souls hence Max's fight is refreshing. This is ironic for Death has become accustomed to his seemingly endless job during this war.