It might even be a core observation about the Götterdämmerung (one that applies to the originary myth, but becomes particularly magnified in Wagner's treatment) that it is, in important ways, about under- and unmotivated actions; about holes in the Reasons Important Things Happen. The Prelude, with its (splendid, tremendous, beautiful) full orchestral articulation of the love between Brünnhilde and Siegfried, sets the tone. There's a stirring emphasis upon the pledging of oaths between the two lovers, but at no point does the text address the fundamental problem: if they are so in love then why is Siegfried buggering off? Handwaving of the 'she sends him on new adventures' or 'he must regain his kingdom' kind won't do here (and aren't actually mentioned in Wagner's libretto) ... why not marry and then do whatever it is Siegfried has to do?
The real answer is that Siegfried has to go because the plot requires it: in order that he can be tricked into drinking the potion of forgetfulness, marrying Gutrune, and so precipitating the tragic sequence of events. But that reasoning can't be incorporated into the logic of the text on the level of actual representation, so we're left with this great gap.
This in turn prefigures the grand finale: the whole world's ending. Why? Notionally because Wotan's staff gets broken (he has details of all his treaties etc upon it). But this is so thoroughly underdetermined as causality for so mammoth an event as the end of everything that all it does is to advertise its own sequential inadequacy. In fact the whole Götterdämmerung thang is a ratio superior elaboration of the same hole in the logic of things that motivates Brünnhilde and Siegfried's parting in the prelude. It's a weighty, operatic articulation of the celebrated metric: what's 6 x 9? 42. "I always thought something was fundamentally wrong with the universe."