The big idea for us to discuss when thinking about Franz is the Grand March of History. The narrator introduces the concept early in Part 3. Franz, the bookish professor, fetishizes the drama of political activism, of revolution, of parades and rebellion, because these things are totally foreign to him. "Franz felt his book life to be unreal," explains the narrator. "He yearned for real life, for the touch of people walking side by side with him, for their shouts" (3.5.5). The narrator gives us more details:
[A]s long as he lived in Paris, he took part in every possible demonstration. How nice it was to celebrate something, demand something, protest against something; to be out in the open, to be with others. The parades filing down the Boulevard Saint-Germain or from the Place de la Republique to the Bastille fascinated him. He saw the marching, shouting crowd as the image of Europe and its history. Europe was the Grand March. The march from revolution to revolution, from struggle to struggle, ever onward. (3.5.4)
This leads us right into Franz's attraction to Sabina:
She was a revelation. She came from a land where revolutionary illusion had long since faded but where the thing he admired most in revolution remained: life on a large scale; a life of risk, daring, and the danger of death. Sabina had renewed his faith in the grandeur of human endeavor. Superimposing the painful drama of her country on her person, he found her even more beautiful. (3.5.22)
Franz doesn't love Sabina; he loves the idea of Sabina. He loves what Sabina represents to him. He "nourishe[s] the cult of Sabina more as religion than as love" (5.11.7).
Which is why Franz's relationship with Sabina is better than ever after she leaves him. "[Franz] realized to his great surprise," explains the narrator, "that he was not particularly unhappy. Sabina's physical presence was much less important than he had suspected" (5.9.18). This is completely in line with what we've seen of his character so far, because Franz "ha[s] always preferred the unreal to the real" – a prime example being the drama and playacting of the Grand March (5.9.19).
So we've seen that Franz's fascination with Sabina is explained by his fascination with the Grand March. But what explains Franz's fascination with the Grand March?
In a way, the Grand March is Franz's way of trying to find weight, meaning, and significance. Because it marches on eternally through time, the Grand March is not plagued with transience the way that our lives are. By joining the Cambodian protest at the end of the novel, Franz hopes to join the Grand March in the attempts of imbuing his life with some of its weight.
Of course, as we know by now, life is necessarily and unbearably light, which means Franz's endeavor is rather doomed. And it doesn't take long for him to realize this:
Franz had the sudden feeling that the Grand March was coming to an end. […] The space where the Grand March was occurring was now no more than a small platform in the middle of the planet. The crowds that had once pressed eagerly up to the platform had long since departed, and the Grand March went on […] without spectators. […] One day it will be reduced to a mere dimension-less dot. (6.20.6)
Franz desperately tries to prove otherwise; "he felt like placing his own life on the scales; he wanted to prove that the Grand March weighed more than shit" (6.22.4). "But man can prove nothing of the sort," says the narrator, because life is necessarily and unbearably light (6.22.5).