If Fed's really the GOAT like you maintain then what's so bad about losing to him, irrespective of age? At the end of the day 17-19 >>>>> 10-23 and you know it.
Exactly. There is never any shame in Novak losing anything to Roger.
Novak is simply losing to the superior player.
There is a match-up issue too.
Novak can handle Rafa because he knows what he is going to get, and is able to execute a countering strategy.
Federer's complete, aggressive game is too much for him to comfortably handle, though. Even when Djokovic wins (he has to be a lot better on the day to ever do this), he never blows him out or beats him comprehensively.
Almost all of Djokovic's victories over Federer these last few years have gone the distance or have involved several very tight sets, while Federer has shut Novak down on several occasions.
This year, they have played five times, with two being straight-sets. Federer won both, while Novak's first win was 7-6 in the third (3-set match), and the other a 5-setter.
Don't get me wrong, Djokovic is more than capable of beating Federer,
but it never comes easily for him. He always looks visibly uneasy when playing Federer these days, because he knows that Federer's style is the perfect counter for his own. He might even have a mental block against Federer like Davydenko did.
He looks much more at ease against Rafa, because he knows that he holds the upper hand in that match-up. He can be sure of Rafa's game plan, and all else being okay, he can usually handle it.
More generally, one of the advantages players like Federer have is that they have so much in their "toolbox" that even when playing their B or C games, they can still win comfortably. Rafa and Novak both have all-time great A games to rival Federer's, but when not firing on all cylinders, they are lucky to make the 3R.