In this poem, remembering seems painful. In the first stanza, it doesn't seem so, especially because the memory described seems to be a good one and the transition from present to past is like watching a "vista"—a beautiful view. However, in the second stanza, we learn that the remembering is...
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?