The poem's scene is of a frosty, sunshiny morning. A beautiful, frail flower dies from the cold, freezing temperature. There is no surprise in the scene because it is reenacted in the beginning of winter every year. There is no remorse on the part of the sun. It is has other work to do. Nature depends on the sun to count off the time in days.
The killing of the flower is unintentionally harsh because nature does not intend to hurt, only to follow its natural plan. God does not interfere with the frost. It is doing what it was supposed to do in his scheme of life; therefore, God approves.