The speaker is trying to tell someone they are dying, or at least that their soul is slipping away, and the person they love isn't listening. Instead of showing concern or surprise, the listener sits and laughs. The speaker reads that laughter as cruel, almost as if the person is smiling "on the Devil's behalf," a striking image that makes the indifference feel more sinister than just rude.
The dying could be literal, but it could just as easily describe a deep emotional collapse, the feeling of being drained by someone who won't notice your pain. The poem doesn't settle this, and that vagueness adds to its weight.
The mood is sad, desperate, and a little frightened. The speaker isn't angry so much as pleading: "I wish you'd stop for a moment." The last lines carry the heartbreak. They still love this person "with all my heart," but they can't go on for long. Love and exhaustion sit side by side, and the short, plain lines make the confession feel raw and unguarded.