I have never fully appreciated unrequited love. Whenever I’ve pursued someone and they’ve given me little or negative responses in return, it’s been all too easy to say, “Well, I guess they aren’t for me after all” and turn my gaze elsewhere. The idea that you could eventually alchemize someone’s indifference into 24-carat affection struck me as (a) a lot of work, and (b) counterintuitive. Wouldn’t you be better off starting from the base of someone who actually likes you? For those who say that there’s no accounting for the capriciousness of the heart, I guess I’ve just been granted a sanguine one that responds more to affection than to infatuation.
I say this because, after Swann’s ironic pursuit of Odette and Charlus’s difficulties with his young proteges, Proust now closes in on the central relationship between Marcel and Albertine, and especially after the preceding thousand-plus pages, it certainly seems like he ought to know better. And he has no one to blame but himself.
We got a glimpse of the trouble during Within a Budding Grove, where he abandoned a fairly rational like of Andree for Albertine’s more difficult, prissy personality. By the end of Sodom and Gomorrah, the relationship is much more serious, yet it’s arisen almost completely in Marcel’s head, much as Swann’s infatuation with Odette did. There are few significant interactions between Marcel and Albertine, and much tossing and turning of the facts in his head. This will change in the fifth volume, but for now, it is a sign of Marcel’s continued detachment from being an aggressive participant in the world around him. Even when he does act–as he does at the end of Sodom and Gomorrah–it is within constraints clearly set by his own mind, and not by society. In part, this stems from the disillusionment he suffered after his obsession with the Guermantes; now he will listen to himself more than the expectations of society. Yet as with Swann, who was stuck with the low-class Verdurin salon sheerly on account of Odette, Marcel’s solipsism places him farther from Albertine than he otherwise would be.
The echo of Swann persists, yet where Odette’s lesbian tendencies had been a marginal aspect of her general amorality, Albertine’s become central in Marcel’s mind very early on. On seeing Andree and Albertine being ambiguously physically affectionate, he is aware of the parallel:

I thought then of all that I had been told about Swann’s love for Odette, of the way in which Swann had been tricked all his life. Indeed, when I come to think of it, the hypothesis that made me gradually build up the whole of Albertine’s character and give a painful interpretation to every moment of a life that I could not control in its entirety, was the memory, the rooted idea of Mme Swann’s character, as it had been described to me. These accounts contributed towards the fact that, in the future, my imagination played with the idea that Albertine might, instead of being the good girl that she was, have had the same immorality, the same capacity for deceit as a former prostitute, and I thought of all the sufferings that would in that case have been in store for me if I had happened to love her. (832)

“if I had happened to love her”: Aware of Swann’s misfortune and uncertain of his own feelings, Marcel is no more able to control himself. He is more self-conscious (in several senses), yet he is no wiser. Thirty pages later, he confronts Alberine about being a lesbian and “the profound disgust I felt for women tainted with that vice” (861), and Albertine says that no, “Andree and I both loathe that sort of thing. We haven’t reached our age without seeing women with cropped hair who behave like men and do the things you mean, and nothing revolts us more.” (862) They then kiss passionately, and Marcel rationalizes away every doubt and every possible comparison between Albertine and Odette: “Was there not a vast gulf between Albertine, a girl of good middle-class parentage, and Odette, a whore sold by her mother in her childhood?” (863) An older voice immediately reflects:

I ought to have gone away that evening and never seen her again. I sensed there and then that in a love that is not shared we can only enjoy that simulacrum of happiness which had been given to me at one of those unique moments in which a woman’s good nature, or her caprice, or mere chance, respond to our desires, in perfect coincidence, with the same words, the same actions, as if we were really loved. The wiser course would have been to consider with curiosity, to appropriate with delight, that little particle of happiness failing which I should have died without suspecting what it could mean to hearts less difficult to please or more highly privileged; to pretend that it formed part of a vast and enduring happiness of which this fragment only was visible to me…I ought to have left Balbec, to have shut myself up in solitude, to have remained there in harmony with the last vibrations of the voice which I had contrived to render loving for an instant, and of which I should have asked nothing more than that it might never address another word to me; for fear lest, by an additional word which henceforth could not but be different, it might shatter with a discord the sensory silence in which, as though by the pressure of a pedal, there might long have survived in me the throbbing chord of happiness. (864)

[Two side notes: first, the unbelievable prescient echo of Wittgenstein in the separation of public discourse from private sensation in the first part of the passage. Second, the insistence on a purely aesthetic apprehension of emotional experiences, as a distancing mechanism from hurt and pain.]
The anticipatory dread of this passage cuts off any chance of seeing the relationship in a sunny light. With Swann we read the detached report of a man deceiving himself; here the effect is so enveloping we live it and the future regret simultaneously.
The Albertine storyline is triggered, again, by a return to Balbec. Though the trip itself is far less revelatory than his first stay there, his arrival engenders a flood of memories and remembered sensations that is one of the best passages in the book, “The Intermittencies of the Heart:”

On the first night…I was shaken with sobs, tears streamed from my eyes. The being who had come to my rescue, saving me from barrenness of spirit, was the same who, years before, in a moment of identical distress and loneliness, in a moment when I had nothing left of myself, had come in and had restored me to myself, for that being was myself and something more than me (the container that is greater than the contained and was bringing it to me). I had just perceived, in my memory, stooping over my fatigue, the tender, preoccupied, disappointed face of my grandmother, as she had been on that first evening of our arrival, the face not of that grandmother whom I had been astonished and remorseful at having so little missed, and who had nothing in common with her save her name, but of my real grandmother, of whom, for the first time since the afternoon of her stroke in the Champs-Elysees, I now recaptured the living reality in a complete and involuntary recollection. This reality does not exist for us so long as it has not been recreated by our thought (otherwise men who have been engaged in a titanic struggle would all of them be great epic poets); and thus, in my wild desire to fling myself into her arms, it was only at that moment–more than a year after her burial, because of the anachronism which so often prevents the calendar of facts from corresponding to the calendar of feelings–that I became conscious that she was dead…At any given moment, our total soul has only a more or less fictitious value, in spite of the rich inventory of its assets, for now some, now others are unrealisible, whether they are real riches or those of the imagination–in my own case, for example, not only of the ancient name of Guermantes but those, immeasurably graver, of the true memory of my grandmother. For with the perturbations of memory are linked the intermittencies of the heart. It is, no doubt, the existence of our body, which we may compare to a vase enclosing our spiritual nature, that induces us to suppose that all our inner wealth, our past joys, all our sorrows, are perpetually in our possession. Perhaps it is equally inexact to suppose that they escape or return. In any case if they remain within us, for most of the time it is in an unknown region where they are of no use to us, and where even the most ordinary are crowded out by memories of a different kind, which preclude any simultaneous occurrence of them in our consciousness. But if the context of sensations in which they are preserved is recaptured, they acquire in turn the same power of expelling everything that is incompatible with them, of installing alone in us the self that originally lived them…I was now solely the person who had sought a refuge in his grandmother’s arms. (783)

This is, of course, an elaboration on one of the earliest and most famous themes in the book, brought up in the madeleine sequence. It’s turned into a darker and more chaotic form here. What was the evocation of an environment is now an evocation of a potentiality of the self, one of many. And this reflects itself in his shifting attitudes towards Albertine, which are as nihilistically inclined as they are dangerous.