In "A New Sith, or Revenge of the Hope", Martin lays out the case for how the true heroes of the Star Wars saga were Artoo-Detoo and Chewbacca. You have to wonder: how, in a galaxy so big, do all of these characters keep running into each other? Well, this is the most plausible answer to that question that I've yet found, in addition to answering a few other issues.
Whether it came about by design or accident, I'm sort of inclined to agree with this theory, because Chewbacca and Artoo seemed to have always been George Lucas's favorite characters. Kinda makes sense that those two are the master link that keeps this entire story together.