The speaker sets out to help a flower grow and throws everything into it. They don't care if snow comes. They call on the light to come and the sky to pour, almost like giving orders, as if wanting something badly enough could make nature obey. The tone is eager and a little desperate.
Then the poem turns. The last two lines, set apart from the rest, deliver the blow in a flat, matter-of-fact way: the flower dies. The speaker never says what went wrong, but the poem suggests that too much light and too much rain, forced on the flower, may have been the cause. Good intentions don't guarantee a good result.
The title, "Fallen angel," hints that the speaker may see themselves as a would-be rescuer who ends up causing harm. The mood moves from hopeful energy to quiet, stunned sadness, and the short, plain lines leave the reader with a sense of loss and regret.