Well, when the question came up on the MentalFloss website, none other than BTTF writer Bob Gale posted his explanation that both he and director Robert Zemeckis came up with during the making of the film!
“For years, Marty was told that Doc Brown was dangerous, a crackpot, a lunatic. So, being a red-blooded American teenage boy, age 13 or 14, he decided to find out just why this guy was so dangerous. Marty snuck into Doc’s lab, and was fascinated by all the cool stuff that was there. When Doc found him there, he was delighted to find that Marty thought he was cool and accepted him for what he was. Both of them were the black sheep in their respective environments. Doc gave Marty a part-time job to help with experiments, tend to the lab, tend to the dog, etc. And that’s the origin of their relationship.”
I just smiled ear to ear…I am so happy right now, just by this little bit of awesome information…It’s like a kid in a candy store.
I have an addiction…not afraid to say it…