One of the authors at mental_floss asked managing editor Jason English a question he couldn't answer, so he asked the readers.

“Is it ever explained why Marty hangs out with Doc Brown? He’s just in Doc’s house to start the movie and it’s just implied that they’re good friends.”

— Brett Savage

Among the many comments at that post was one from Bob Gale {wiki}, one of the co-writers of the three Back to the Future movies, who helped shape the history of all the characters. His explanation got its own post today. Link