Doc is Marty — from the future! Time travel is invented and Marty, a high school drop out working as a custodian at a Chronolabs facility (a company that peddles in time traveling services), steals one of these time travel devices. He uses this device to commit “time-crimes” such as going back in time to place sports bets. He is eventually discovered and is sought out for his crimes. He manages to escape to the past, hiding out in the least likely location — his old hometown where he takes on an assumed identity as “Doc” a strange and reclusive mad scientist…

Orrrr… The ORIGINAL Marty was Doc’s lab assistant. They built a time machine together, went back in time, altered the future, and ultimately Lab-Assistant-Marty was returned back to 1985.
But there was one problem! 1955 Doc knew that the Lab assistant Marty would be returning to Hill valley again in 1985, which poses a problem because a NEW Marty McFly was about to be born in 12 years! He knew he had to make sure that when a kid named Marty McFly was born, by whatever means necessary he had to get that kid to get in a time machine on the exact night the lab assistant Marty would be returning.
But this new Marty was raised by a different George and Elaine, and unlike his original version didn’t end up being interested in science at all. This Marty, the Marty we all know and love, was interested in Rock and Roll instead.
So to make sure the RockandRoll-Marty would go back in time on November 5th 1985, the Doc built a giant guitar amplifier as incentive for RockandRoll-Marty to hangout with him and become his friend. He let the Lybians know exactly where he would be to make sure that Marty would get into the Deloreon and try to outrun them , drive 88 miles and hour, and ultimately go back in time, avoiding the paradox and continuing the cycle or Marty’s.
That’s why Doc stressed to Marty that he had to be there on the EXACT time with the camera and that’s why at the end of the film he repeated the experiment again, knowing that it would have disastrous results, because he HAD to to avoid a paradox.
Anyone who didn’t see that is retarded. Amirite? However right

plus, doc remembered marty from when he met him back in 1955.