Change and decay run through this short poem like a current. The river is a picture of time: it carries everything along, and even stones, which seem permanent, wear down and vanish. The speaker then turns to "we," and finally to "you," so the idea of mortality moves from a general fact to something pointed and personal.
The middle section sounds weary and darkly wry. The suggestion that you might become "a prisoner in love with their guard" reads like a bleak survival tip: if you learn to love what confines you, life may hurt less. Searching for "the spirit in the bone" promises only disappointment, as if the speaker doubts there is anything lasting beneath the body. The sickness, which the poem never names, is "indiscriminate" and permanent.
The mood is cold, resigned, and quietly bitter. The repeated lines, with only small changes, feel like a verdict, and ending on "You are no exception" leaves the reader with no comfort.