The gist of my too-long ramble below is there probably is, at least definitively within the film, no answer to the "Why?" of Matteo, and that Nicola as he ages increasingly embraces the status quo. Blame inevitable age, if you like, but it doesn't happen to their mother.
Way back, among some finished and unfinished novels that migrate to each computer I buy but that otherwise I no longer bring myself to look at, is one in which my Myshkin-like protagonist lies up a minor tragedy to cause his various acquaintances who have nothing to do with one another to show up at one place at the same time. The Russian Myshkin couldn't lie, of course, but mine did. Just to see, I guess: a sort of wish-fulfillment, curiosity that killed the cat or made the student's mind viable or kept the oldster's young and nimble. Lying is storytelling is lying. The act of writing allows, or allowed, me, playing god just like that character of mine, to introduce to one another versions of people I known but whose paths never crossed otherwise. A tragic, now late, slow guy from work somehow occupies my vacation-vacant condo and hits it off with my garrulous but uneducated father. That kind of thing. Or, as Myshkin, I grab one or another misplaced acquaintance, long gone well before my mountaineering days, and march her way above the timberline to some icy col only to play at not knowing the way back down while sweet panic and thunderheads well.
I don't want to get into most La Meglio gioventù's maybe merits, the pacing and rhythms gained by extraordinary length, the credible aging and acquisition of Time's wisdom, whether for growth or stagnation, in most the characters, the relatively quiet interpolation of passing history. But the sadly heavy-handed, grossly unnecessary insertion of Matteo's "ghost" into the closing scene between Nicola and Mirella made me realize what an orgy of wish-fulfillment the second half is. Suddenly, seemingly because Life or Time heals but maybe just so the filmmaker could resolve things for us, nearly everyone meets everyone. Even at six hours, a story's tick needs its tock. Whether or not director Giordana and his writers intended the nearly dead center division, Part One ticks, and then Part Two tocks.
If this is more than just a really, really good soap opera, then there have to be ambiguity, loose ends, disappointments that come more from life's constraints than from audiences' need for reassurance and resolution. Why, other than to create suspense, does Nicola deny Giorgia's need be involved in finding Mirella? He's suddenly so uncharacteristically lacking in empathy. If he's hurting over Matteo, then he's selfish. If he's just being professional, then we're denied his rationale. I can easily dream up justifications within Nicola's character, but, watching, I sensed a filmmaker struggling to twist an ending out of events that in reality must fractal along indefinitely. With possible exceptions of Nicola's mother and perhaps his too-young-yet, barely formed though marriageable, daughter, Mirella seems the sanest creature in the film, Fate's gift to this at least slightly askew family. Has Nicola even told her about Giorgia, and in what terms? Inevitably she will meet Giorgia (Tiny piece of a six-hour sequel?!), but the film denies us the experience.
Matteo's so ever-present, and all the more so as the film progresses, in no small part because of Giorgia's obsession, that that bit of magic realism offended me as a viewer. Don't plaster on the screen what's already there: don't double images. Besides, I'm not at all certain the living Matteo would have been so generous. The sequence suggests suicide's a path to peace of mind, a return to balance or to freedom from self (What?! Does Matteo reach Enlightenment? Aurrgh!), but no one can know this. Are the filmmakers saying that Matteo by self-abnegation, self-destruction, restored balance to his family? Probably not, but how cruel the suggestion! Even so Matteo's the glue here, the MacGuffin, a key of some sort. I've sat through enough director Q & As to suspect there may be no explanation. Ambiguity's a storyteller's tool. Ambiguity, red herrings, in stories not by genre mysteries cue ineptness from hacks, but deliberation from pros. Matteo's case abounds in red herrings. Despite the transvestite, I don't think he's gay (all I saw in the episode was self-loathing, and nowadays they would have simply told us so; the day he first flirts with Mirella, despite his holding back, he seems no less enthralled than Nicola will be later). Despite the cop stuff, the look, the gun, I don't think he's political (they could have involved him against Nicola's terrorist love but don't; Nicola though, by surviving so wholeheartedly within the status quo does come off political). Surely each of us, as I think the filmmakers intend, will find a different key to Matteo. Empson, writing about ambiguity, usually says not that one or another reading is "it," but that all are present and in play. Matteo's mystery allows the filmmakers' artifice to breathe. If I see any clue, it lies way back, the day or two with Giorgia, when she, if not he, knew him already a soul broken in some way that her experience had taught her to understand. Whatever was wrong with him likely was already wrong then. I don't know what it was, but banishing from the film's close the first and only character who "got it" seems to doom the whole family, if only in the sense of their not "getting it" about Matteo and Giorgia. It seriously undercuts that silly ghost-Matteo scene.
I know too little to speak but recall vaguely that there may be a genetic tendency to suicide: sort of a no-fault clause.
Luigi Lo Cascio as Nicola, incidentally, looks like both Jean-Pierre Léaud and Dustin Hoffman, but, even saying that, I can't see that those two ever behave quite alike.