Paris, years earlier, as Marcel (the narrator and the author are nearly undifferentiable in this section) recounts the story of Swann’s unpleasant affair with Odette, a not-terribly-deep woman who is as incapable of returning his affections as she is of understanding them. So say Proust and Swann, in baroque language Odette probably couldn’t understand. Odette herself plays up her ignorance, calling herself “an ignorant woman with a taste for beautiful things.” Swann, unsurprisingly, falls for her and confounds himself with jealousy, and generally makes himself miserable over her long after she’s lost interest. This goes on for a while.
Despite the fact (it’s presented as objectively as can be) that Odette is beneath him, intellectually and socially, Proust presents Swann’s attraction to her as explicable, if only because the explanation takes up a good chunk of the two hundred pages of “Swann in Love.” Despite some editorializing that makes it clear that Proust is most in sympathy with Swann, the sympathetic air mostly arises from the description of the tiny details that captivate Swann, that keep his obsession going even as it wrecks his social standing.
It’s not a rational response that Swann has, obviously, but I’ve rarely read such a detailed itemization of the particulars that cause an irrational response. Part of this is my own bias: I’ve never been interested in stories that grow out of two characters’ de facto attraction to each other and consist of little but the misery they make for each other. Maybe I’ve been lucky to avoid experiences that would make me empathetic. Wong Kar-wai’s Happy Together (the title is ironic) spends two hours showing a couple repeatedly breaking up and getting back together as soon as they forget why they broke up. They forget fast. The movie bored me. So the “hopeless love” angle isn’t one that I’m going to touch.
Elias Canetti’s Auto-da-Fe spends five hundred pages talking about how a bookish older man and his library are destroyed by his incredibly base housekeeper, whom he falls in love with and marries. It’s supposed to serve as a symbolic representation of nihilism destroying the cultured mores of the educated classes, but the book doesn’t work: the characters aren’t convincing, and the relationship less so.
Proust is better than that. Swann is foolish, he’s arbitrary, and he’s inconsistent, but for all the details, he seems sui generis; there’s never been someone who fell for another person quite in the way he does, though I’m sure some have come fairly close. Even when Proust goes off into theorizing, it is always about the particulars of Swann’s pathology, not about the sorrows of humanity. So Swann is as large to readers as he is to himself. There is so much detail about his particular tastes and preferences, particularly his attention to a single, small passage of music that he comes to associate with his love for Odette, that he’s not just another “mad love” character out of Ariosto behaving stupidly, but someone for whom his every action is justifiable, and someone whom I find reasonably comprehensible. He makes a mental image of that passage, knowing little about music or even about the piece’s overall structure:

He had before him something that was no longer pure music, but rather design, architecture, thought and which allowed the actual music to be recalled. This time he had distinguished quite clearly a phrase which emerged for a few moments above the waves of sound. It had at once suggested to him a world of inexpressible delights, of whose existence, before hearing it, he had never dreamed, into which he felt that nothing else could initiate him.

The phrase is not just music but is itself sui generis, as is Swann himself, as presented. The arbitratiness of his tastes, as with the choice of musical notes, is akin to the establishment of a private reality, not just the whims of an aristocrat. Whether his tastes are explicable is not meaningful, since they govern him like natural laws.
This approach accomodates one other crucial thing, which is inconstancy. Generally, even in someone like Flaubert, when a character changes a core opinion, it’s presented as a fulcrum, something that tips the balance and causes the novel to progress. That doesn’t happen here; inconstancy is made part of Swann’s being. He changes his mind about Odette several times; he gets fed up with her, he falls back in love with her, he allows himself to forget what she’s done, he replaces her with his ideal. The changes are rapid, but the constantly (and drastically) evolving mental processes of Swann don’t change much in his relationship with Odette; it’s only by the microscopic examination of his thoughts that you’re aware that his opinions are changing as much as they are. This could come off as arbitrary or inexplicable, but again, rationality is not the order of the day. Natural law is.