For some reason, likely the expression on her face when they join hands, I imagine this song coming from Narcissa's point of view. You know, like, it's her way of reflecting back on the life of her friend. She & Lucius must've been around at school to have seen his relationship w/ Lily, so she likely knew he never stopped loving the girl. She probably knew about his abusive childhood, too. She sometimes gets overlooked as a character, so it's nice to imagine something like this. Just my opinion.