Jessea L. Vaughan's speaker feels invisible. The "box" is probably not a literal one. Judging by the imagery, it stands for isolation, a place the speaker is shut inside while life carries on nearby. The people in the poem laugh, fight, and eat, ordinary things that show how close they are and how little they notice. The speaker sits "in the corner of the room," unseen and unmissed.
The most painful moment comes when the speaker imagines what the others would find if they looked: "what's left / Of a friend they never had." That line suggests a loneliness that goes deeper than being ignored, since the speaker has never had real connection at all. Even time loses its shape. Minutes and hours blur, and the speaker claims not to care, though that flat tone reads more like numbness than calm.
The repeated phrase "with no one" works like a heavy refrain. The mood is bleak, quiet, and resigned.