partly because he saw that Nole was not afraid of him, or in awe of him like most players on the tour.
that's your way of seeing Roger as the bad guy, but I heard other things from guys working for sponsors : Novak told them "see this guy : I will have kicked him off in one year".
Call it "not being afraid of him" if you want,
but that's more aggressive than "not being in awe of him" as you say with the typical vision some fans of other players have from Fed as full of conceit enjoying others bending
Let's say that their story started in conflict, both with what happened in DC 2006 (I agree that Djokovic's health problems have been underrated by many ones but Djokovic admitted that he faked sometimes in that time, notably against Monfils in the US Open, it's not something which was entirely wrong)
and that way Djokovic put himself in frontal competition with him from the start.