Michael asked Harry if he loves Susan on the second page of the book and forces Harry to say it out loud. There is also this beauty of language:
"Harry Dresden," he said. "You of all people, should know the power of words."
Then, in the last 50 pages of the book, Harry finally tells Susan he loves her, which is the key to enabling him to break Lea's spell and prevent Susan from going full vampire.
"I love you." Why it worked right then, why the webbing of my godmothers' spell frayed as though the words had been an open flame, I don't know. . . . I'll say this though: Some words have a power that has nothing to do with supernatural forces. ... Those three words are good ones.
Several chapters later we see it was of so much importance to Harry to have said the words "I love you" that he was content to die having said them.
"I love you," I said, or tried to say to Susan. "I love you." She pressed her mouth to mine. I think she was crying. "Hush" she said. "Harry. Hush. I love you , too." It was done. There was no reason to hold on.
Finally, the book ends with Susan sending a birthday card that only has three words.
I'll let you guess what three.
In sum, the start and end of the book is about the importance of Harry telling Susan he loves her and the power that those three words have. If Harry doesn't say I love you, he doesn't save Susan (she probably bites him and gets killed by the poison - he passes out and gets killed by Kravos, but Justine does CPR saving him and helping him get his power back). He then kills Bianca and the vampires out of revenge instead of because he is willing to wage war to save Susan. That is the first step down the dark road.
