Four short lines, and one tangled idea: the person the speaker loves is both necessary and harmful. The comparison to oxygen is the key. We can't live without it, yet the poem pushes the image somewhere odd, suggesting oxygen is "painlessly, slowly killing" the speaker. That nods to the idea that breathing, living, and simply being alive wear us down over time.
The speaker seems to feel trapped in a kind of devoted dependence. Staying close means a gentle, almost unnoticeable decline. Being apart would be far worse: sudden, painful, final. The contrast between "slowly" and "instantly" makes the choice feel lopsided, so the speaker stays.
The mood is quiet, resigned, and a little dark. There's no shouting or drama, just a calm admission that love can be both a lifeline and a slow wound. The scattered spacing of the lines adds to this, making the poem feel like a hesitant, held breath.