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Rereading from the beginning and with how much information now, I've come with a theory. The old man, who I assume is ML, is reading a crumpled and old letter surrounded by fireflies. Maybe they had gotten married on the island and decided to never leave, however she had died young from her illness. Now ML is reading her last letter and remembering the days the spent together. So it's a pretty tragic end, real bittersweet.