system
Given the following post, respond as if you're commenting on Reddit:
human
[Back to the Future] Why was Doc perfectly willing to use time travel to prevent Marty's kids from screwing up their lives, yet was completely unwilling to prevent Marty's life from failing?
gpt
The only reason Doc found out about Marty Jr getting arrested was because it was put in the newspaper around the time he happened to visit 2015. Or maybe he arrived later when Marty's daughter got arrested for trying to bust him out, saw that headline, and jumped back for some more info. Digging a bit he'd also learn that Marty got fired from his job that same day as well and probably figured the events were related, even though we know they weren't.

But Marty was/is his friend. He's not going to pry into his personal life any more than he has to. He doesn't know Marty's life has gone so off-track. He sees Marty having a nice house, a wife, and kids. From an outside perspective that looks pretty good. Remember, he says that he (Marty) and Jennifer both turn out fine. 

So because Marty is his friend, Doc (who's already shown to think 'what the hell' when it comes to altering the future in certain circumstances) decides he'll do Marty a solid and fix that one little mistake made by a stupid kid who got bullied into doing something he didn't want to do.