Almost every line here is a question, and the questions sound like grief looking for something to push against. The speaker watches someone fade: they ask for less, hear less, say less, and wear a strange fixed smile. The speaker seems to want a grand ending, with a brave gesture, a calm stance, some wisdom to pass on, but gets none of it. What they get is gentler and harder to accept: the person is held and kissed as they "slide softly away."
The feeling underneath is bewilderment, and maybe a little anger. The last lines make this clearest. The speaker imagines the dying person's usual "roar and thunder" and says it would have burst out if the situation were reversed, if it had been "one of us" dying. That suggests the speaker is mourning the loss of a forceful presence, and struggling to see how such a person could go so quietly.
The mood is hushed, tender and unsettled. Short, plain lines leave space for silence, and the unanswered questions leave us in that silence too.