In "The Dead," why does Gabriel suddenly feel passion for his wife? What triggers it?

"The Dead" is the final story in James Joyce's "The Dubliners," a collection of short stories about Joyce's beloved Ireland, an Ireland he felt was crippled by its "spiritual paralysis." In the introduction to this book, Terence Brown of Trinity College in Ireland writes,

...the detail of Joyce's art is not simply the realist's involvement with a congeries of fact as a reflection ...of a world which palpably exists before and after the act of literary composition...but the strategy of a symbolist who believed that the given in the hands of an artist would speak its own radiant if disturbingly uncomfortable truths about the world....In Dubliners Joyce chooses to re-embody the details of a Dublin life he knew intimately  in a context ...to compose an interpretative statement about the city as a whole.

In Joyce's story "The Dead," the living are not in control; the setting is a night for the ghosts.  Gabriel himself is the son of the dead sister Ellen of his two aunts.  His conversation with Molly Ivors is on the subject of the dying language of the Irish, Gaellic and the loss of interest in the Irish culture. Symbols such as the handing of a coin to the housekeeper by Gabriel  suggests the pettiness of their lives.

As the guests leave Julia and Kate Morikan's annual holiday party, Gabriel does not go to the door with the others; instead, he sees his wife leaning on the banisters of the stairway as she listens to a distant singing voice (Mr. D'Arcy's).

He asked himself what is a woman standing on the stairs in the shadow, listening to distant music, a symbol of.

Caught in the aesthetic of the moment, Gabriel imagines that if he were a painter he would paint a picture in lights and shadows of his wife and call it Distant Music. When someone closes the door, he can better hear and notices that the song is in "the old Irish tonality and the singer seemed uncertain both of his words and of his voice."  [This suggests the lost of Irish heritage.]

After this experience, "Moments of their secret life together burst like stars upon his memory" and his passion for his wife is rekindled.  When they return to their hotel room, Gariel wishes to express his passion for his wife, but, ironically, she tells him of her "moments of a secret life" in which a young man loved her passionately and even died for her unrequited love.

When Gabriel learns that Greta is in love with a dead man, and not him, he experiences the Joycean "epiphany":

He saw himself as a ludicrous figure, acting as a pennyboy for his aunts, a nervous well-meaning sentimentalist,...and idealising his own clownish lusts, the pitiable fatuous fellow he had caught a glimpse of in the mirror.

Realizing that he cannot love Greta as her boy did, his condescension to her earlier is replaced by an admiration, but Gabriel feels himself entering the world of the dead--"moving westward"--the secrecies of consciousness where the world seems a mere trace, a shadow with the incertitudes in the dead.