Two people look at each other and see two very different things. He sees her as a goddess, someone who holds up the whole sky: sun, moon and stars. She sees him as a best friend, and nothing beyond that. The poem is about that gap between them, a love that isn't returned in the same way.
The speaker doesn't stand inside either person's feelings. Instead, the poem reports them calmly, like a quiet observer. That calm is what makes it sting. The first line is long and glowing, stretched out with its grand image, while the ending shrinks into short, blunt pieces: "No more / No less." Those two lines sound fair and even, but they also close a door. Her feelings aren't unkind; they're simply not the same size as his.
The mood is wistful and a little sad, but gentle. Nobody is blamed. It's just the ache of caring more than the other person does.