The speaker starts by saying "no," as if correcting someone, and the correction is a playful one. "I love you to the moon and back" is a familiar way to say how much you care, but the speaker points out a flaw in it: going to the moon and back describes a round trip, and a round trip has an end. For a love that "knows no end," that phrase is too small.
So the poem swaps it for something bigger. In the last stanza the speaker says "yes" and stretches the love past the moon to "the galaxies beyond it." The word "yes" makes it feel like the speaker has won a small argument with themselves.
The mood is warm, confident, and a little cheeky. The casual voice, with lowercase lines and the relaxed "gonna," makes the declaration feel spoken, not rehearsed. It reads like someone who is certain of their feelings and slightly amused that the usual words can't hold them.