see you auntie said:
"I Don't Give A Shit Where The Stuff I Love Comes From. I Just Love The Stuff I Love." - Patton Oswalt

This couldn't be more applicable then right here. As far as I'm concerned Chewie is Han's Co-Pilot/ Best Friend/ Partner in Crime. That's it, it was more than evident on screen.

And this is also a good reason why I strongly dislike the EU (and to a certain extent you can add the PT to this too), because you don't need to explain or have explained to you every little detail from the OT. There's a reason it wasn't apart of the original movies because chances are it was either contrived or really boring.

Couldn't agree more.

There was a discussion going on in another thread about the 'Star Wars Encyclopedia', which is apparently ful of this sort of backstory and contains explanantions for every little thing (Did you know the Sarlaac is actually one of many, that came down from space in the form of spores.... more importantly, did you give a fuck?).