The speaker starts with the biggest question there is, living or dying, and admits that in the long run it makes no difference. Then the poem turns on the word "But." Even if nothing lasts, the present still counts, and the speaker wants it to count.
The repeated "Let it matter" works almost like a plea. It moves outward from "me" to "you" to "them," so the wish grows from a personal need into something shared. The short, bare lines make it feel like someone thinking aloud, not building an argument.
In the second stanza the speaker brushes aside the usual worries: meaning, legacy, being remembered. "Who cares" sounds defiant, but it's also gentle. The last line lowers the bar to its simplest point: if something matters to just one person, that is enough. The mood is quiet, hopeful, and a little tender, a small act of resistance against feeling that nothing matters.