I agree with BS and TM above, Didi. We used to wonder somewhat about what kept Fed motivated when he really had won everything, bested so many records, and yet love of the game, of winning, maybe stretching his lead over Rafa in Slams kept him motivated enough to win Wimbledon and get the #1 back. Rafa also loves to win, and he loves clay to play on clay. As to RG in particular, he's got Pete's (and Roger's Slam) record(s) as motivation. Plus, he seems to relish his rivalry with Djokovic, and beating him there will always be a carrot on the stick, IMO. If he had no real rival for it, he might take his eye off the ball, a bit, though not likely enough to lose, since there would be no decent rival.
That said, I believe that, when there was no real difference between Nadal and Federer in the 08 Wimbledon in the 5th, Rafa, never having won it, and having had his heart broken the year prior, perhaps did want it that bit more that pushed him over the edge. If Novak and Rafael get to a better epic next year, or whenever, and there's not much to choose between them, it could be the desire for the goal not-attained that pushes Djokovic over the finish line. In that case, I could subscribe to your theory that hunger might trump a whiff of jade.