unwelcome, that the narrator remembers "In spite of [him]self" rather than willingly. The singer's masterful singing is "insidious"—harmful, treacherous—because it compels the memories to return. The narrator is "Betray[ed]" by the singing, and his heart "weeps" to feel suddenly a part of

old Sunday evenings at home, with winter outsideAnd hymns in the cosy parlour, the tinkling piano our guide.

The memories that rush back are so lovely, so warm and inviting, so rich with an innocence that can never return that it doesn't matter how well the singer, now, sings. The speaker can only remember "The glamour / Of childish days" and "weep . . . for the past." We often seem to recall memories from our childhood and reflect on those years as more innocent and carefree, as possessing a beauty that life as an adult simply lacks. For the narrator, this makes remembrance dangerous, like a "flood" that drowns, that offers the rememberer no choice in where he goes; he drifts back through his memories without control. It is ironic, isn't it, that the loveliness of the memories is precisely what makes the remembering so painful?

This poem is incredibly poignant and sad it its treatment of memory.  In the poem, the speaker hears a woman singing to the accompaniment of a piano, and it brings back memories of his childhood, when he would sit underneath a piano in his family’s parlor, “pressing the small, poised feet of a mother who smiles as she sings.”  Lawrence writes almost with animosity, referring to the music which sends the mind back in time as “insidious,” and the speaker as being “betrayed” by his memory.  And as he is in the throes of remembrance and nostalgia, the speaker’s adult life is cast in depressing shadow as, he says, “I weep like a child for the past.”

Through the use of stimulus and memory, “Piano” hits at the very heart of the disconnect between childhood and adulthood; how as one gets older, nothing will ever be as sweet or as comfortable in life as one’s childhood.  These past events have a “glamour,” Lawrence writes, and harbor a belonging that it is rare to find outside one’s happy childhood home.  And as we get older, our past becomes more distant, which makes us long for it all the more.

In this poem, the speaker recalls his childhood. In the present time, the speaker is listening to a singer accompanied by piano music. In the present moment, he is an adult man. The song evokes memories of his childhood because it reminds him of his mother singing at the piano. He is mentally transported back to his experiences as a child sitting under the piano while his mother (presumably) played and sang. 

Note that in the second stanza, he is transported to this memory in spite of himself. As a rational adult, he seems to resist such a romantic, sentimentalized longing for childhood. As the speaker moves into the third stanza, he says that by this time, the song of the present time is not so relevant anymore. And this is because he has been fully transported (emotionally, mentally) back to his childhood. His emotions have overrun and conquered his adult, rational mind and sent him back to the past. That's why he says his "manhood is cast . . . " Initially, he thinks it is irrational to be conjured to this overly sentimental memory, but the emotion is too much and overrides this resistance: 

The glamour

Of childish days is upon me, my manhood is cast

Down in the flood of remembrance, I weep like a child for the past.