The story of ‘Back to the future’ is about a young Marty McFly being forced to travel back in time to escape some terrorist who had killed Doc and were trying to kill Marty.

Marty gets himself trapped in the past and has to get the Doc of that time to help him return back to the present. The whole time Marty is trapped in the past he tries to warn Doc about the future and to make sure he is prepared. But Doc doesn’t want to know about his future, he doesn’t want to know as this might have dramtic affect not only on his destiny but others around him.

So after Marty succefully returns to his own time with the help of past Doc, he arrives a couple of seconds before he is forced to travel back in time.
He then rushed over to where Doc was on the grown where he had been shot.

And finds that Doc wasn’t dead but had a bulletproof vest and the letter that Marty had written for him back when he got stuck. By Marty giving Doc the option to know information about his personal future gives him the opertinuty to know his destiny and have the power to change it if he were to read the note otherwise he would follow the predetermined end that was shown at the start of the film.