Hard-core fans of Back to the Future got a bit of a raw deal. Where the Star Wars nerds and Trekkies each had at least half a dozen films to go with countless book spinoffs, TV series, and comics, we had only the original BttF trilogy, a few video games, and a better-left-forgotten cartoon series. (Yes, I know there are also three novelizations —they hold a proud spot on my bookshelf.) With such a paucity of content to pore over, there's little discord within the fan community and relatively few mysteries to argue about. But there are a few: Is the bum named Red in 1985 the same person as Mayor Red Thomas from 1955? (He's not.) Why does Marty's paternal great-great-grandmother from Part III look so much like his mother? (Is there a history of incest in the McFly family?) And, most vexing of all, why would a crazy old scientist be friends with a teenager?
It was apparently a slow news day on Wednesday, because one of the stories making the rounds was that there was finally an explanation for the friendship between Doc Brown and Marty McFly. After the question was posed on Mental Floss, a commenter claiming to be Bob Gale, co-writer and producer of the trilogy, gave the somewhat mundane explanation that a young Marty had heard Doc was dangerous, so "being a red-blooded American teenage boy," he sneaked into his lab, and they somehow became friends as a result.
First off, no one on the Internet knows you're a dog, or whether you're really Bob Gale. To be fair, the explanation given on Mental Floss isn't far off from what the real Bob Gale said on Facebook in April, but I've seen enough Internet hoaxes to say I'll wait for a picture of him holding a sign that says "Yes, it's me" with today's newspaper before I buy it completely. (Update, Aug. 19, 2011: OK, so here's Bob Gale holding a sign that says, "Yes, it's me." But where's my newspaper?)
Why else might Doc and Marty have become friends? An early draft of the script (available online) provided a quick explanation that Doc hired Marty to sweep his garage, and an even earlier draft (also available online) had the two running a video-pirating business together. Other drafts also had Doc building the time machine out of a refrigerator and Marty returning to a present-day where Doc had made a fortune building robot servants that run on Coca-Cola—so these rejected scripts shouldn't be taken as gospel.
Some of the commenters at Mental Floss brought up a competing theory, that Doc was friends with Marty because he remembers meeting him in 1955. But this doesn't seem credible, since according to the rules of space-time physics established within the film—namely that alternate realities are spawned by time travel, and there's no Terminator-esque loops—the story begins in a timeline where Marty hasn't traveled back to the 1950s, as evidenced by the fact that his parents are still losers. The version of Doc we meet at the end of the movie was indeed part of the revised timeline, and thus remembered Marty perfectly well—although he probably had a few awkward years where he had to pretend that he didn't. (As for Marty's parents, they must have been surprised when their son grew up to look exactly like the guy who got them together.)
In the late-1990s, there was a rumored prequel in the works called Back to the Beginning, which involved a young Marty becoming friends with Doc. Bob Gale (the real one) has denied that he had anything to do with this project, if it even existed at all, noting that a Back to the Future film that took place before Doc had invented the time machine would be pretty boring.
Personally, I never found the relationship between Doc and Marty that implausible. Friendships between local scientists and young boys have been firmly set in the public consciousness since at least the days of Watch Mr. Wizard. An early draft the film called Doc's character "Professor Brown," but producer Sid Sheinberg demanded that it be changed—presumably because this boy-professor bond was already a cliché, having been parodied in Peabody's Improbable History, Police Squad! and other programs.
Still, according to Wikipedia, Gale and director Robert Zemeckis had trouble making Marty and Doc's friendship seem believable until they added the scene where Marty plays guitar on the giant speaker. Wait, why did Doc have that giant speaker in his laboratory? Did Marty go in there and turn the knobs all the way up at 8 a.m. on a regular basis? I love overanalyzing a movie as much as the next guy, but I'm perfectly happy with the explanation that Doc and Marty are friends because they are.