Six short lines, and almost every one hangs on its own, as if the speaker is hesitating while they talk. The poem is about a belief most of us hold: that out there, somewhere, is one person made exactly for us, and that this person will stay for good.
The key word is "imagine." The speaker doesn't say such a person exists. They say we love to picture it. That small choice puts a gentle distance between the wish and the reality, and it hints that the idea may be more fantasy than fact. There's no anger or bitterness here, though. The tone sounds closer to quiet tenderness, maybe a little wistful, as if the speaker understands the hope and half shares it.
The vague words "somehow," "somewhere" and "someone" make the dream feel hazy and out of reach. Then "forever" lands alone on the last line, sounding both hopeful and a bit too big to be believed. The mood is soft, longing and faintly uncertain.