The speaker begins almost overflowing with happiness. Their spirit is too big for their skin, and their heart seems to leap out of bed, laughing. A real, honest smile is described as something rare, a delicacy, and the line "If only it could stay" already hints at worry. Then a single careless word, spoken by a "naïve tongue," seems to break the mood, and the smile dies.
From there the poem falls, just like the rollercoaster in the title. The speaker drops into a cave where the light is swallowed up. They may climb out eventually, but they expect the darkness to return and "devour" them again. The repeated "Up, down, up, down" mimics that motion, and the speaker's radiance rises and falls with it.
The mood shifts from bright and giddy to heavy and tired. The final line, "Not all rollercoasters are fun," is flat and blunt, and it reads like a quiet admission that these swings in feeling are exhausting rather than thrilling.