system
Given the following post, respond as if you're commenting on Reddit:
human
[Back to the Future] Why is Doc so against Marty changing the future if he both lets Marty improve his father's (and his own) life and invests in a bulletproof vest for when the Libyans come?
gpt
Marty preventing his parents from getting together was the bigger change. Doc was just trying to get Marty to change things back as much as possible. As for the Libyans, he changed his mind at some point in the intervening thirty years.