Spike has secrets. Secrets he isn't proud of. Secrets which clog up his thoughts to the point of needing to shout them out... This is why he has a diary. He converts all rage, hate, worry, fear, embarrassment and even crushes to simple text on a page. But this has continued to the extent that he is paranoid of anypony reading a single word of it...
So what happens when the diary mysteriously disappears?