According to the Eye of Beljoxa, Buffy's resurrection triggered the rise of the First Evil, a revelation which would have presented an alternate means of winning the war, if anyone had thought of it. What if someone did?
With the right witches to make it happen, time travel should be simple: Go back. Change only what needs to change. Act normal. But Buffy doesn't travel light. The weight of her loves, losses, hopes, and regrets goes wherever she does. And so does her vampire.
When Buffy brings her mom and sister to Spike to protect from the hell god, Glory, he sees a chink in her armor—she needs him, she trusts him... maybe, just maybe, she doesn’t hate him as much as she claims. Will that boatload of manly responsibility the Slayer’s dropped into his lap save Spike from a life of lonely desperation?