How we cite our quotes: All quotations are from Back to the Future.
Quote #4
Doc: Please, Marty, don't tell me, no man should know too much about their own destiny.
This pretty much sums it up. Even if there is information that Doc might deem valuable, nay, life-saving, he still doesn't want to know it. Luckily, he changes his mind about that later.
Quote #5
Doc: No, Marty, we've already agreed that having information about the future could be extremely dangerous. Even if your intentions are good, they could backfire drastically. Whatever you've got to tell me I'll find out through the natural course of time.
Here Doc focuses on the "intentions" part of it. He recognizes that Marty wants to do good by divulging information about the future, but there's a divide between intent and reality. He knows that no good can come of it… until he does a little re-evaluating after ripping up that note.