Memory and ineffability
Proust recalls the the ineffable qualities of his memories. In this book, we see a narrator who does not simply catalogue moments for trivia's sake. Instead, there are deep, poignant mysteries locked within each passing moment, and suddenly a smell or coincident will send him back in time, to re-experience moments that he had long forgotten. By remembering these moments and examining them, he rediscovers that the moments are mysterious, and that sometimes, the beauty of a moment seems absolutely unexplainable and divine.
True love
True love is the subject of this prose in many ways, many of them not as obvious as others. To say there is something ineffable about a passing moment might actually be the same as being in love with that moment, and that dilemma—that the timing of social events could lead to feelings of deep attachment and sincerity between people—he wonders often about the nature of such coincidental moments. In addition to this, he clearly remembers being stunned by a woman's beauty, many times, and he wonders if those memories are permanent because of true love, or something else, like nostalgia.
Ennui and nostalgia
To find the present unimaginably boring and to find the past unrealistically blissful is to find one's self in "nostalgia." This author speaks volumes about nostalgic reasoning, euphoric recall, but without ever explicitly mentioning his present suffering and agony. But they are signals about his depression, or at least of his existential boredom. The narrator's focus on the past is predicated on unsolved problems within his present existence, but those themes come through most in accidental ways, because of the conversational tone of the prose, and because the narration moves along in a sort of train-of-thought. The effect is simply that Proust delivers a wistful, painfully tragic artistic statement.
