The speaker sets up a simple rule: "hello" is a way of opening up, and "goodbye" is a way of giving up. Everything in the poem follows from that. If the speaker ever says goodbye, they mean it, and you shouldn't expect to see them again. If they say hello, it means something real: you are "someone I need."
What stands out is how plain and exposed this feels. There's no elaborate imagery, just short lines that sound almost like a warning mixed with a confession. The speaker admits a need, but also shows a kind of guardedness, since goodbye is treated as a final door closing. The title, with its "Goodbye! Goodbye?", hints at doubt, as if the speaker is testing the word and fearing it.
The ending turns into a plea: "So please don't ever say goodbye. Never." The mood is tender and anxious, a quiet fear of losing someone, wrapped in a stubborn insistence that some relationships shouldn't end.