Spike feels like a Dragon without purpose and without anything to offer his friends. He leaves Ponyville without warning his friends, in an attempt to find his purpose on his own.
Princess Luna finds Spike and shows him the nightmares that his friends have been having in his absence, in an attempt to convince him to return to Ponyville of his own free will. Spike learns that dreams are the quickest road to the heart.
EDIT: 1000+ views! Thank you all so much. I consider this work to be my magnum opus, and I'm glad to see that it received some attention!