The title and nineteenth-century framework suggest a great social novel centered on the decline of an upper-bourgeois mercantile family.
As often happens with great books, as one delves deeper into the reading (and we imagine the same occurred for the author while writing it), another theme emerges: the unstoppable force driving toward dissolution, decay, and death.
To express the novel’s profound meaning, Mann makes brilliant use of music.
The adolescent Hanno, the last descendant of an entrepreneurial dynasty, plays “one of his improvisations” on the piano.
He plays a motif

extremely simple, (...) a short-breathed invention, a meager invention, which nonetheless acquired a strange, mysterious, and significant value

.
At first the motif remains in the background, submerged by “bustling runs,” vigorous, as if urging on to “ever new efforts.”
Then the first

sweet, wandering, tormenting motif, that first mysterious motif

, takes over.

The tide of surfacing cacophonies fell upon her, gathering threateningly, surging forward, retreating, climbing, sinking, and straining once more toward an ineffable goal that had to arrive, to arrive at once, in that very instant, at the dreadful climax where the yearning torment had become intolerable.
And it arrived: it could no longer be stopped, the convulsions of longing could be prolonged no more; it came as if a curtain were torn aside, as if gates were flung open (...) it was the motif, the first motif!

What brought about the end of the Buddenbrooks was not ruinous speculations, the squandering of wealth through disastrous marriages, or competition from rival merchant families; it was an internal decay, a weariness, a slow yet inexorable loss of the founders’ primal energy.
We are in Lübeck, a Hanseatic city on the Baltic Sea, and the story unfolds between 1835 and 1877.
The firm of Johann Buddenbrook (founded in 1768) is at the height of its wealth and prestige.
The novel centers on the siblings Antoine and Thomas.
The first three hundred pages recount how the young Antoine, clever and restless, renounces love out of loyalty to the “firm,” in order to contribute through a marriage of convenience to the family’s good name and prosperity.
Already betrothed to a Hamburg merchant, she falls in love with a young man, but upon reading the thick ledger recording the Buddenbrook genealogy, she feels

like a shiver, (...) like a link in a chain, her father had written.
Yes, precisely as a link in that chain, she held great importance and responsibility: she was called upon to contribute with deeds and resolve to the history of the family

.
Antoine unhesitatingly devotes herself to being the vestal virgin of the family, the custodian of an upper-bourgeois decorum.
Then emerges the figure of Thomas, who reaches the pinnacle of the social ladder with his election to the city senate.

Thomas Buddenbrook (...) showed in his face and carriage a pronounced sense of dignity; yet he was pale, and his hands especially (...) were as white as the cuffs emerging from his black sleeves, with an icy pallor that betrayed how cold and dry they were.
Those hands, whose well-tended oval nails bordered on a bluish tint, could assume at certain moments, in certain postures of convulsive unconsciousness, an indescribable expression of hostile sensitivity and almost morbid reserve—an expression that hitherto had been alien and ill-suited to the rather broad, bourgeois, even if shapely, hands of the Buddenbrooks

.
From his very appearance, the inner suffering of Thomas is evident—one for whom shouldering the firm and the family is a duty, not a pleasure.

He felt a void in his soul.
(...) But his need for action, his restlessness, his activity had been fundamentally different from the tenacious, steady industriousness of his fathers: an artificial thing, then, a craving of the nerves, a narcotic, (...) a martyrdom

.
Now ill, Thomas confesses to his sister how he has grown increasingly fond of the sea.

Long waves rolling in and breaking, one after another, endlessly, aimlessly, wild and desolate

.
His sister falls silent:

but one does not say such things! she thought, looking away so as not to meet her brother’s eyes

.
And so, when in its final pages the novel descends into long descriptions of the deaths of Thomas and Hanno, with sorrowful scenes of the few surviving women, helpless and stunned by the tragedies that have destroyed the Buddenbrooks, we are not surprised; for we are well aware that the vitality, the sense of sacrifice, and the allegiance to a great dynasty have all withered away.

The anticipation of death lies at the center of Thomas Mann’s world, and this novel is no exception.
At first, death seems distant, for prosperity fosters an atmosphere of placid bourgeois serenity; only the depictions of the characters and their surroundings are soft, ambiguous, decadent.
And yet we are in the triumphant nineteenth century! More than the plot, it is the writing that skillfully, subtly conveys the sense of decay.
Then, past a certain point, the narrative becomes explicit: the accounts of agony and illness (even of visits to the dentist) are meticulous, attentive to every physical and moral suffering; the settings turn dull, gray, joyless; the characters grow ever sadder.
It is neither a social nor a psychological novel; the context always lingers in the background, the characters are drawn through their physical appearance without delving into their emotions or existential conditions: they are brushed in rather than delineated.
It is the writing itself that speaks to us, creating the atmosphere, conveying the overall meaning of the novel.

Why read it? It is long, at times redundant, but one must let oneself be carried by the words.

Buddenbrooks: The Decline of a Family