Spike had always had a pivitol roll in the survival of ponies, as his simple existance pushed forward anything and everything the world over. His dreams of ponies, of sun, and sky. His dreams of rocks and water.
Post-Secret of My Excess, Spike runs off from Ponyville, trying to distance himself from the Ponies who he could have hurt. The real question, is how did he end up thinking the Changeling Hive was any better?